Ulster, situated in the northeastern portion of the island of Ireland, was one of the major Irish kingdoms of the medieval period. It was annexed by England in 1461, and the Irish nobility was forced to swear allegiance to the English Crown. Ongoing Irish hostility resulted in the Nine Years’ War (1594–1603), in which an allied Spanish fleet sacked Kinsale, port city on the southern coast of Ireland, before England ultimately suppressed the rebellion. The leader of the rebellion, Hugh O’Neill, earl of Tyrone, was pardoned and agreed to work for the English Crown. In 1607, he and other leaders of the rebellion fled into exile, abandoning their large estates. The English government parceled their land to caretakers willing to undertake the settlement of the lands, leading to the creation of widespread English and Scottish settlements throughout the counties of Armagh, Cavan, Donegal, Derry, Fermanagh, and Tyrone, known collectively as the Ulster Plantation. There was naturally great hostility on the part of native freeholders and tenants, whose rights and traditions were frequently violated. When the systematic settlement foundered, independent and individual migrations from England and Scotland created a more fragmented settlement, mixing English, Irish, and Scottish agriculturalists. Between 1605 and 1697, it is estimated that up to 200,000 Scots and 10,000 English resettled in Ireland. Most settlers in the early stages were poverty-stricken Lowland Scots. Starting in the 1640s, however, an increasing number of Highlanders joined the migration. About 10,000 Highlanders had been sent to suppress a rebellion in 1641, and many stayed on, eventually bringing their families. The descendants of these Lowland and Highland, mostly Presbyterian Scots, are known as Scots-Irish.
More than 100,000 Scots-Irish from Ulster immigrated to America between 1717 and 1775, mainly because of high rents or famine and most coming from families who had been in Ireland for several generations. In the colonial period, these Scots-Irish were usually referred to simply as Irish and represented the largest movement of any group from the British Isles to British North America in the 18th century. Together with a large emigration from Scotland itself, this movement laid the foundation for a strong Scottish ethnic component in the cultural development of both the United States and Canada (see Scottish immigration). In the U.S. census of 2000 and the Canadian census of 2001, more than 9.2 million Americans and 4.1 million Canadians claimed either Scottish or Scots-Irish ancestry.
With the Anglo-Irish Treaty of 1921, the island of Ireland was divided, creating a state within the United Kingdom called Northern Ireland, which comprised six of the nine counties of historical Ulster: Antrim, Armagh, Down, Fermanagh, Derry, and Tyrone. The remainder of the island became the Irish Free State, a dominion under the British Crown. Though not exactly coterminus with either the old medieval kingdom or the Ulster Plantation, Northern Ireland is sometimes referred to simply as Ulster.