Although more than a quarter of the population of Ireland in the eighteenth century was Protestant, the Anglo-Irish Anglicans formed a minority of this number. It was the Ulster settlers and their descendants, overwhelmingly Presbyterian, who were in the majority. The Penal Laws, designed as they were to protect the privileges of members of the Church of Ireland, disenfranchise and discriminated against Presbyterians as well as Catholics, though the effects for the Presbyterians were mitigated to some extent by their superior economic strength and the tight-knit communities in which they lived. Nonetheless, to a people who had fled Scotland originally to escape religious persecution, the impositions of the Penal Laws were intolerable. This was reflected in the increasing radicalization of Ulster opinion, which was to reach its peak in the rebellion of 1798, but also in emigration from Ulster to America. This was the start of a process which had far-reaching consequences. Up to then, the movement of peoples had been into Ireland. Now began the long exodus.
As well as political discontent, this first movement of emigration also had economic causes. The majority of Ulster Presbyterians were poor small holders, artisans, weavers and labourers, and these were most vulnerable both to the succession of natural disasters - crop failures, smallpox epidemics, livestock diseases - which recurred throughout the eighteenth century, and to the increasing commercialization of Ulster, with the constant efforts of landlords to increase the profitability of their lands by raising rents. The increasing importance of the linen trade was also influential, and the numbers of emigrants rose and fell as this trade prospered or faltered. The very nature of the business facilitated emigration, since the ships which brought flax seed from America often returned with a cargo of emigrants. Before 1720, the stream of migrants across the Atlantic was slow but steady, with New England the favoured destination. After that date, the rate of emigration grew, with a peak in the late 1720s, and a decline in the 1730s, when relative prosperity returned to Ulster. The famine of 1740-1741 gave a sharp impetus to the renewal of emigration, which rose steadily through the 1760s, when more than 20,000 people left from the Ulster ports of Ne wry, Port rush, Belfast, Lares and Ferry. The migration reached a climax in the years 1770 to 1774, when at least 30,000 people departed. Over the course of the whole century, it is estimated that more than 400,000 emigrated from Ulster, the vast majority to North America; in 1790, the number of the United States population of Irish stock has been estimated to have been 447,000, two-thirds originating from Ulster.
Those who left were mostly indentured labourers, contracting to work for a number of years for employers in colonial America in return for their passage, with very few convicts or independent travelers. One important result, significantly different from later Catholic emigration, was the fact that the move was often effected by entire families or even communities, allowing the settlers to maintain their way of life in the new world, and providing a continuity of religion and tradition in keeping with the religious and cultural separateness they had already brought with them from Scotland to Ireland. To point up this separateness, in America they called themselves "Scots-Irish", and the distinctive culture they maintained allows us to trace their settlements in the United States with some precision. Initially, most of the emigrants sailed to the Delaware estuary, especially to Pennsylvania, where Cumber land County became the effective centre of the Scots-Irish settlement. In the 1730s, a second wave of emigrants, accompanied by the children of earlier settlers, moved farther west in Pennsylvania and south into the Valley of Virginia. By the 1750s, a third movement pushed further south again into the Carolina and Georgia back-country, where they met and mixed with emigrants arriving through southern seaports such as Charleston and Savannah. By the 1790s, more than half the settlers along the Appalachian frontier were of Ulster lineage. The influence of their culture, their music, religion and way of life, can still be seen in these areas today.
The blend of Protestant evangelism, fierce self-sufficiency and political radicalism which many Ulster Presbyterians brought with them to the New World, was powerfully influential in the American Revolution. In all of the states, but especially in Pennsylvania, New Jersey, New York, Delaware and Maryland, the immigrant Scots-Irish and their descendants played a role in the war out of all proportion to their numbers; as an officer on the British side put it, "call this war by whatever name you may, only call it not an American rebellion; it is nothing more or less than a Scotch Irish Presbyterian rebellion". After American independence, the Scots-Irish tradition continued to play an important part in American life; the great nineteenth century steel-producing town of Pittsburgh was created by Scots-Irish entrepreneurs, and their representatives are found at all levels of American society, in the professions, industry, finance, education, and with presidents such as Andrew Jackson and Wood row Wilson.
Ulster radicalism also found another outlet, at home. Throughout the eighteenth century, local secret societies such as the "Hearts of Steel" and the "Hearts of Oak" had sprung up, dedicated to defending their members, generally the poorest tenants. This tradition, along with the influence of the French and American Revolutions, provided the background for the United Irishmen, an organization dedicated to republican ideals and incorporating Catholic, Presbyterian and Anglo-Irish radicals. On May 23rd 1798, a rebellion against British rule organized by the United Irishmen broke out, with risings in Meath, Carlow, Children, Wicklow, Dublin and Wexford in the East, Antrim and Down in Ulster, and, with the assistance of a French invasion, in Mayo in the West. Over the course of the next six months, the rebellion was crushed by the British, at great cost to lives and property; over 30,000 people died, and a million pounds worth of property was destroyed. The failure of the 1798 rebellion resulted in increased pressure for the union of Ireland and Britain. The subsequent abolition of the Irish self-government was one of the reasons for the huge growth in emigration in the nineteenth century.
Many of the post-1798 refugees went to the United States and took with them the ideals of fraternity and equality which had inspired the defeated United Irishmen. A large number of these emigrants rose to political and social prominence in the New World.