Scotch-Irish continue to move west but maintain deepest cultural imprint
in southern Appalachia, Ohio Valley, and the Deep South (Mississippi, Alabama) 

Scotch-Irish continue to move west but maintain strongest cultural imprint in southern Appalachia, Ohio Valley, and the Deep South (Mississippi, Alabama), Northern Louisiana, East Texas, and Oklahoma.

"Greater Appalachia," from Colin Woodard, American Nations: A History of the Eleven Rival Regional Cultures of North America

Later developments:

In literature "Okies" (or people living in or deriving from Oklahoma) appear most prominently in the poor farm families of John Steinbeck's The Grapes of Wrath (1939).

"Okie," Wikipedia: In the 1930s in California, the term (often used in contempt) came to refer to very poor migrants from Oklahoma (and nearby states). Jobs were very scarce in the 1930s, but after the defense boom began in 1940, there were plenty of high-paying jobs in the shipyards and defense factories.

The "Okie" migration of the 1930s brought in over a million newly displaced people; many headed to the farms in California's Central Valley.

By 1950, four million individuals, or one quarter of all persons born in Oklahoma, Texas, Arkansas, or Missouri, lived outside the region, primarily in the West. The core group of Okies are descendants of Scotch Irish who display a marked individualistic political bent. During 1906-17 a good amount joined the Industrial Workers of the World, and Okies tended toward left-populism in the 1930s.

(Orange County, California)

While there had been Scottish immigration to America in the 1600s, it was not until 1700 that it began in numbers.  Between 1707, when Scotland and England combined to form the British Union, giving the Scots legal access to all of the colonies, and 1775, when the American Revolution began, Scottish immigration soared.  Immigration paused during the Revolution, but resumed after the fighting ended.  Scots immigrated to American in three distinct groups:

Lowland Scots: Assimilated to English ways, the Lowland Scots were primarily skilled tradesmen, farmers, and professionals pulled by greater economic opportunity in America.  They usually immigrated as individuals or single families, then dispersed in the colonies and completed their assimilation to Anglo-American ways.

Highland Scots: More desperate than Lowland Scots, the Highlanders responded primarily to the push of their deteriorating circumstances.  In 1746 the British army brutally suppressed the Jacobite Rebellion in the Highlands and Parliament outlawed many of the Highlanders' traditions and institutions, creating much discontent.   At mid-century, the common Highlanders also suffered from a pervasive rural poverty worsened by the rising rents demanded by their landlords.  The immigrants primarily came from the relatively prosperous peasants, those who possessed the means to emigrate and feared a fall into the growing ranks of the impoverished.

After 1750, emigration brokers and ambitious colonial land speculators frequented the northwest coast of Scotland to procure Highland emigrants.  The brokers and speculators recognized that the tough Highlanders were especially well prepared for the rigors of a transatlantic passage and colonial settlement.  Preferring cheap, if dangerous lands, the Highland Scots clustered
in frontier valleys, especially along the Cape Fear River in North Carolina, the Mohawk River in New York, and the Altamaha River in Georgia.  By clustering they preserved their distinctive Gaelic language and Highland customs, in contrast with the assimilating Lowland immigrants.

Scotch-Irish (or Ulster Scots): Nearly half of all Scots immigrants came from Ulster, in Northern  Ireland, where their parents and grandparents had colonized during the 1690s.  Like the Highlanders, the Scotch-Irish fled from deteriorating conditions.  During the 1710s and 1720s they suffered from ethnic violence with the Catholic Irish, a depressed market for their linen, the hunger of several poor harvests, and the increased rents charged by grasping landlords.  The Ulster immigration to the colonies began in 1718 and accelerated during the 1720s.

The Scotch-Irish or Ulster Scots immigrated in groups, generally organized by their Presbyterian ministers, who negotiated with shippers to arrange passage.  Once in the colonies, the Scotch-Irish gravitated to the frontier where land was cheaper, enabling large groups to settle together.  Their clannishness helped the immigrants cope with their new setting, but it also generated frictions with the English colonists.  Feeling superior to the Catholic Irish, the Ulster Scots bitterly resented that so many colonists lumped all the Irish together.  In 1720 some Ulster Scots in New Hampshire bristled that they were "termed Irish people, when we so frequently ventured our all, for the British crown and liberties against the Irish Papists [Catholics]."  As a compromise they became known in America as the Scotch-Irish.

(materials below edited from "MCCLELLAND GENEALOGY" at http://homepages.rootsweb.ancestry.com/~mcclell2/homepage/migrate.htm)
The following is abstracted from James G. Leyburn, The Scotch-Irish, A Social History (Chapel Hill: U of North Carolina P, 1962), which covers the migration of lowland Scots from Scotland to Ireland beginning in 1610, then to America in the 1700's, and finally across the mountains to the Pennsylvania frontiers and down the valleys into Virginia and the Carolinas. This abstract sketches the waves of migration from Ulster to America. There were five great waves of emigration--1717-18, 1725-29, 1740-41, 1754-55, and 1771-75  with a lesser flow in intervening years--which provide, in effect, a chart of the economic health of northern Ireland.

1717-18. This first movement, so significant as a path-opener, had as its immediate cause the years of drought; but it was the opinion of Archbishop King and Dean [Jonathan] Swift that not even the dire effects of bad crops and high prices would have been enough to make the people move if they had not had the added goad of rack-renting*, still such a novel practice that it caused intense resentment. In a letter of 1718 to the Archbishop of Canterbury, King summed up the causes and tried to persuade his colleague to use his influence to arouse the English conscience to a realization of the effects of what was happening. He charged: "I find likewise that your Parliament is destroying the little Trade that is left us. These & other Discouragements are driving away the few Protestants that are amongst us. ...No Papists stir except young men that go abroad to be trained to arms, with intention to return with the Pretender. The Papists being already five or six to one, & a breeding People, you may imagine in what conditions we are like to be." . . .
    In a sense, the emigrants of 1717 would be explorers whose report on their experiences could guide those who came after. The Ulstermen who went to Boston found unexpected difficulties and a welcome that lacked warmth. Those who followed them in the next two years were made to understand that they were not at all welcome. The people who entered America by the Delaware River, on the other hand, found a land of the heart's desire. Their enthusiastic praise of Pennsylvania persuaded others to follow them, and then still others, until by 1720 "to go to America" meant, for most emigrants from Ulster, to take ship for the Delaware River ports and then head west. For the entire fifty-eight years of the Great Migration, the large majority of Scotch-Irish made their entry to America through Philadelphia or Chester or New Castle.

[*Rack-rent was simply raising the rent on the land after the period of the lease had expired, and renting to the highest bidder. Lease terms in Ulster were usually 31 years, much longer than they had been in Scotland, and were reasonable in the 17th century. As more immigrants came in and land became scarce, landlords could get more for use of their land. However, the dispossessed, who had been there for a generation or two, were outraged.]

1725-29. The second wave was so large that not merely the friends of Ireland but even the English Parliament became concerned. Parliament appointed a commission to investigate the causes of the departures, for they had reached proportions that portended a loss of the entire Protestant element in Ulster.
    Letters from immigrants themselves spoke of rack-rents as a determining cause of this second wave; but the Pennsylvania Gazette mentioned these as only one of the "unhappy Circumstances of the Common People of Ireland" that had resulted in so great an exodus. An article in that journal (November 20, 1729) reported "that Poverty, Wretchedness, Misery and Want are become almost universal among them; that . . . there is not Corn enough rais'd for their Subsistence one Year with another; and at the same Time the Trade and Manufactures of the Nation being cramp'd and discourag'd, the labouring People have little to do, and consequently are not able to purchase Bread at its present Rate; That the Taxes are nevertheless exceeding heavy, and Money very scarce; and add to all this, that their griping, avaricious Landlords exercise over them the most merciless Racking Tyranny and Oppression. Hence it is that such Swarms of them are driven over into America."

1740-41. Famine struck Ireland in 1740* and was certainly the principal occasion for the third large wave, which included numbers of substantial Ulstermen. An estimated 400,000 persons died in Ireland during 1740-41; for the next decade there was a tremendous exodus to America.
     This third wave marked, on the American side, the first movement of Scotch-Irish in any numbers beyond the confines of generous Pennsylvania to the southwest. Following the path through the Great Valley, many Ulstermen now went into the rich Shenandoah Valley of Virginia, whose southern extremity opens out toward North and South Carolina. Arthur Young, writing in 1779, estimated that between 1728 and 1750 Ulster lost a quarter of her trading cash and probably a quarter of her population that had been engaged in manufacture. His comment, if accurate, suggests the caliber of men now leaving the country.

[*Not to be confused with the potato crop failure that was the cause of the great Catholic Irish migration in 1845-47.]

Shenandoah Valley

1754-55. The fourth exodus had two major causes; effective propaganda from America and calamitous drought in Ulster. A succession of governors of North Carolina had made a special effort to attract to that province colonists from Ulster and from Scotland. That two of these officials were themselves Ulstermen lent persuasiveness to their invitation and appeal. As drought ravaged the countryside, testimony of Scotch-Irish success in American struck a particularly responsive chord in hearts back home. . . .
    At this moment, however, the Scotch-Irish pioneers had their first taste of real trouble with the Indians. The French and Indian wars broke out in the colonies and were to last for more than seven years. For the time being, these violent disturbances effectively dried up the source of new immigration. More than this, Ulster was just now undergoing a true economic recovery. Her prosperity was so pronounced that the vacuum left by emigrants began to be filled by arrivals of people from the south of Ireland and from Scotland. Her population began to increase apace; indeed, it was the pressure of numbers, combined with a new economic depression, that caused the final large wave of migration.

1771-75. Young, writing in 1779, when the outbreak of the American Revolutionary War had eliminated the possibility of further emigration, said that the people of Ulster had by 1770 become very poor, living chiefly "on potatoes and milk and oat bread," and that their little farms had been divided and subdivided until "the portions were so small they cannot live on them." More than this, the shipowners at the ports of Belfast and Derry were in distress because their "passage trade, as it was called," which had long been a regular branch of commerce, was now cut off.
    There was, however, a special reason for the departure of this final wave. In 1771, when the leases on the large estate of the Marquis of Donegal in county Antrim expired, the rents were so greatly advanced that scores of tenants could not comply with the demands and so were evicted from farms their families had long occupied. This aroused a spirit of resentment so intense that an immediate and extensive emigration was the consequence. During the next three years nearly a hundred vessels sailed from the ports in the North of Ireland, "carrying as many as 25,000 passengers, all Presbyterian." Froude gives an even larger figure: "In the two years which followed the Antrim evictions, thirty thousand Protestants left Ulster. ...

Throughout the fifty-eight years of the Great Migration, religious liberty had been a motive only at the beginning. It is nevertheless significant, both for Ireland and America, that those who left Ulster were almost all Presbyterians. Members of the Established Church rarely went, nor did Roman Catholic Irishmen. ...

All of the thirteen original American colonies received Scotch-Irish settlers. By comparison with the main stream that flowed through Pennsylvania, the Valley of Virginia, and the Carolina Piedmont, however, Scotch-Irish settlement in other colonies was insignificant in numbers. The strength of Presbyterianism in many of the colonies (New Jersey, for example) was not, as might be supposed, evidence of Scotch-Irish settlement, on the contrary, most of these churches had been founded by English and Welsh Presbyterians and many by immigrants directly from Scotland.

A clear distinction should be made at this point between colonists from Scotland and those from Ulster, for the two have often, to the complete distortion of events, been thought identical. It has already been noted that by 1717 Scots and Ulstermen were two different nationalities. Extensive emigration from Scotland to America occurred during the eighteenth century, possibly a fourth or a fifth as large as that from Ulster; but the reasons for Scottish emigration were distinct. Before the union of the two Crowns in 1707, many Scots were exiled as criminals and many more came as indentured servants or as merchants to America. After the Union, since Scots had equal rights with Englishmen, including the right of moving to the colonies, thousands came over to escape the grinding poverty at home. Defeat of the Highlanders in 1746, after the collapse of the Stuart cause, with the determination of the government to "civilize" these people, caused a large exodus; and the enclosure of lands, the dispossession of tenants, and the consequent dissolution of ties of personal loyalty binding man to chief, sent thousands of others to America. The pull from the colonies was, as usual, the opportunity for a better life. At times during the nineteenth century there came to be a positive "rage for emigration" throughout both Lowlands and Highlands.

Scots in America from the first showed traits clearly different from those of the Scotch-Irish. Scots were seldom explorers, Indian fighters, or frontier traders; they played only a minor role as pioneers, preferring to settle in the east and to carry on business enterprises. Their greatest difference from their Ulster cousins, however, was seen at the time of the American Revolution: whereas the Scotch-Irish were usually ardent patriots and notable fighters in the cause of the colonies, the Scots were, with notable exceptions, Loyalists faithful to the Crown. Only in their Presbyterianism and a few of their traits of personality did they resemble the Scotch-Irish. In North Carolina the Highland Scots for a long while retained their Gaelic language and even their Highland dress.

Children and grandchildren of the original Scotch-Irish settlers in America were always among the leaders in the move to the new West; but they were no longer Scotch-Irish in their social characteristics and outlook. Just as they were likely to become Methodists and Baptists instead of remaining Presbyterians, so they were likely to marry persons whose background may have been English or German. The memory of Ulster and its respectabilities and distinctions meant little or nothing to these constant pioneers. They were Americans.

[The Scotch-Irish] moved immediately upon arrival to a region where there was neither a settlement nor an established culture. He held land, knew independence, had manifold responsibilities from the very outset. He spoke the language of his neighbors to the East through whose communities he had passed on his way to the frontier. Their institutions and standards differed at only minor points from his own. The Scotch-Irish were not, in short, a "minority group" and needed no Immigrant Aid society to tide them over a period of maladjustment so that they might become assimilated in the American melting pot. Like all people, whether immigrants or stay-at-homes, they must have known individual discouragement and disappointment; some may even have had a heightened feeling of inner loneliness, a quality of mind Weber attributes to most Calvinists who reflect upon the implications of the doctrine of predestinatiion. But to the extent that their neighbors shared similar experiences and attitudes, without pressure from other Americans to be different, the Scotch-Irish were not . . . marginal men. They were, on the contrary, full Americans almost from the moment they took up their farms in the back-country.

The course of Presbyterianism in American between 1717 and 1789 neatly reflects the transformations of the mind and the social life of the Scotch-Irish as they became Americans. The eighteenth century in the colonies was a period whose currents of thought had inevitable effects upon church as well as state. Presbyterianism changed much during the century, and in three aspects its changes were significant for the Scotch-Irish: the church became Americanized; it enlarged its conception of service to the common man; and it made tentatives toward democracy. Yet during the very century that saw its increase in vision and effectiveness, the Presbyterian Church lost its hold upon thousands of Scotch-Irish for whom it had been a birthright. ...

In spite of all the expansion of education and the remarkable missionary accomplishment, the church could not begin to meet the religious needs of the two hundred thousand Scotch-Irish, who by 1776 were filling the back-country and steadily increasing their large families. Certainly the church was vividly aware by now of the spiritual needs of the people. Presbyteries ordered pastors to leave their congregations to make missionary journeys among the settlements--preaching, performing marriages, administering the sacraments, consoling the ill and bereaved. Young men who wished to enter the ministry were not ordained until they had visited the frontier. So persistent were the calls that a good part of the time of each Presbytery's meetings was taken up by consideration of appeals from Scotch-Irish settlements. Yet the Church's best was not enough: thousands of the Scotch-Irish people were without the care of a church or minister, and had been for years.

What Presbyterians could not do, Baptists accomplished. All the ardor and adaptability displayed by the former following the Great Awakening [George Whitefield's evangelism in colonial America beginning in 1738] could not overcome the major obstacle of insufficient numbers of ministers. One fundamental Presbyterian commitment stood in the way: the clergy must be well educated. Baptists had no such requirements. To them the gospel was simple, uncomplicated, within the reach of all. Neither Christ nor his disciples had been university men, and his final command had directed ordinary persons to preach the gospel to all men. More than this, it required no complex organization to form a Baptist church; the approval of no Presbytery or other ecclesiastical court was involved. A group of like-minded Christians could form a congregation and select as their minister a dedicated Baptist who felt the "call." He was forthwith a minister, endowed, as he felt, by God's grace to perform all the functions of his office. While Presbyterians were spending six years or more at great expense getting ready to preach, Baptists were already at work--and more of them every year.

At times the zealous young Baptist ministers and missionaries and exhorters could not even read or could read only haltingly, but they knew many passages of the Bible from memory and could speak directly to the hearts of their ready listeners about the great issues of life and death, sin and hell, faith and heaven. ...

Late in the eighteenth century the Methodist Church, reflecting the zeal of the Wesleys and the far-sighted direction of its first American bishop, Francis Asbury, began to share the Baptist success. After independence, when the Appalachians began to be traversed and the Ohio Valley to be filled, the progress of these two denominations was accompanied by methods truly sensational. Whitefield's meetings may be said to have been forerunners of the "revival meeting," which both Baptists and Methodists eagerly adopted; but by 1802, in Kentucky, the revival had lead to the still more fervid and dramatic "camp meeting.." The two sects were evangelical and assiduous in a way that no Protestants had ever been before.

The Methodists devised one of the truly effective adaptations to frontier conditions of life, the circuit rider. A minister, instead of being tied to a single church, rode hundreds of miles each month to visit pioneers on their remote farms. If there were neighbors, he would preach; in any case, he could perform all the services of a pastor to a scattered flock, comforting, counseling, marrying young couples, burying the dead. The devotion and indefatigability of these circuit riders became proverbial: Kentuckians remarked of a day of foul weather that no one would be abroad in it "but crows and Methodist ministers."

Irish Diaspora