County Donegal Irish: Contae Dhún na nGall. Sometimes unofficially known in Irish as Tír Chonaill) is a county located in the west of the Province of Ulster, in the northwest of Ireland. It is one of three counties in the Province of Ulster that does not form part of Northern Ireland. It is the most northern county in all of Ireland, and is part of the Republic of Ireland. County Donegal is the fourth largest county in Ireland and the largest county in Ulster. The name 'Donegal' comes from the Irish, meaning 'The Fort of the Foreigners'.