Kingdom of Ulster
Ulster was a wide area with varying boundaries in Northern Ireland from ancient times until the 12th century Norman invasion of Ireland, when it was incorporated into the English Lordship of Ireland as the Earldom of Ulster. In its early times it consisted of vast swathes of land inhabited by the Ulaid race. However during the 4th and 5th centuries, much of it broke away becoming Airgíalla and Aileach, leaving what is today county Down and county Antrim.
Despite its reduced size, Ulster would play an important historical role, setting up an Irish colony on land conquered from the Picts in Scotland called Dalriada. This covered todays Argyll in the Highlands and once breaking lose from Ulster, formed the base for the conquest (and extermination) of the native Picts, setting up the initial Kingdom of Alba. The men of Ulster also held for a time the Isle of Man.
Today Northern Ireland is frequently referred to as Ulster (i.e: the Ulster Unionist Party).