A former kingdom of Ireland, lying in the north-east of the island. The kingdom of Ulster reached its zenith in the 5th century ad, at the beginning of the Christian era. During the Anglo-Norman conquest the de Lacey and de Burgh families held the earldom of Ulster from 1205 to 1333. By the 16th century, the O’Neill clan had reasserted its commanding position in the area, until the failure of the Earl of Tyrone’s rebellion against Elizabeth I and her unwelcome religious policy (1594–1601) marked the end of O’Neill supremacy. James I promoted the plantation in Ulster of thousands of Presbyterian Scots and Protestant English and many Catholics were forced off their land. These Protestants supported William III in his campaign against James II, which culminated in William’s victory at the Battle of the Boyne (1690). At the division of Ireland in 1920–21, six of the nine counties that originally comprised Ulster opted for self-governing status as Northern Ireland, a province of the United Kingdom. The remaining three counties of Cavan, Donegal, and Monaghan became part of the Irish Free State (later the Republic of Ireland).