Why Cavan, Monaghan, and Donegal Are Not in Northern Ireland

These three Ulster counties—Cavan, Monaghan, and Donegal—are part of the Republic of Ireland because of the Anglo-Irish Treaty (1921). During partition negotiations, the British government insisted on a six-county Northern Ireland (Antrim, Armagh, Down, Fermanagh, Londonderry, Tyrone) to ensure a Protestant/unionist majority. The remaining three Ulster counties had Catholic/nationalist majorities and were included in the Irish Free State (now Republic of Ireland). They have never been part of Northern Ireland.