Ulster formed one of the historical provincess of Ireland. Two thirds of Ulster (comprising the counties of Antrim, Armagh, Londonderry, Down, Fermanagh, and Tyrone) currently form part of the United Kingdom of Great Britain and Northern Ireland. The Unionist part of the population of Northern Ireland frequently refers to this area as 'Ulster'.

The three Ulster counties of Cavan, Donegal and Monaghan are part of the Republic of Ireland. The combined population of these three counties in 2002 was 246,571.

In the 1600s Ulster functioned as the last redoubt of the traditional Gaelic way of life, and following the defeat of the Irish forces at the battles of Kinsale (1601), Mountjoy and Dunboy Castle, Elizabeth I succeeded in subjugating Ulster and all of Ireland.

The name Ulster also appears in some other contexts originally named in honour of the Irish province:

An ulster is a kind of overcoat.