At various times in its history, it has been known as County Tirconaill, County Tirconnell or County Tyrconnell (Irish: Tír Chonaill). The former was used as its official name during 1922–1927. This is in reference to both the old túath of Tír Chonaill and the earldom that succeeded it.
County Donegal was the home of the once mighty Clann Dálaigh, whose most well-known branch were the Clann Ó Domhnaill, better known in English as the O’Donnell dynasty. Until around 1600, the O’Donnells were one of Ireland’s richest and most powerful native Irish ruling families. Within Ulster, only the Uí Néill (known in English as the O’Neill Clan) of modern County Tyrone were more powerful. The O’Donnells were Ulster’s second most powerful clan or ruling-family from the early 13th century through to the start of the 17th century. For several centuries the O’Donnells ruled Tír Chonaill, a Gaelic kingdom in West Ulster that covered almost all of modern County Donegal. The head of the O’Donnell family had the titles An Ó Domhnaill (meaning The O’Donnell in English) and Rí Thír Chonaill (meaning King of Tír Chonaill in English).