County Donegal ( /ˈdʌnɨɡɔːl/ or /ˌdʌnɨˈɡɔːl/; Irish: Contae Dhún na nGall or Contae Thír Chonaill) is a county in Ireland. It is part of the Border Region and is also located in the province of Ulster. It is named after the town of Donegal. Donegal County Council is the local authority for the county. The population of the county is 160,927 according to the 2011 census.

In terms of size and area, it is the largest county in Ulster and the fourth largest county in all of Ireland. Uniquely, County Donegal shares a border with only one other county in the Republic of Ireland – County Leitrim. The greater part of its land border is shared with three counties of Northern Ireland: County Londonderry, County Tyrone and County Fermanagh. This geographic 'isolation' from the rest of the Republic has led to Donegal people maintaining a distinct cultural identity and has been used to market the county with the slogan Up here it's different. While Lifford is the County Town, Letterkenny is by far the largest town in the county. Letterkenny and the nearby city of Derry form the main economic axis of the north-west of Ireland.

Donegal or Donegal Town ( /ˈdʌnɨɡɔːl/ or /ˌdʌnɨˈɡɔːl/; Irish: Dún na nGall) is a town in County Donegal, Ireland. Its name, which was historically written in English as Dunnagall or Dunagall, translates from Irish as "stronghold of the foreigners" (i.e. the Vikings).

Donegal gave its name to County Donegal, although Lifford is now the county town. Until the early 1600s, Donegal was the 'capital' of Tír Chonaill, a gaelic kingdom controlled by the O'Donnell Clan of the Cenél Conaill. Donegal sits at the mouth of the River Eske and Donegal Bay, which is overshadowed by the Bluestack Mountains ('the Croaghs'). The town is bypassed by the N15 and N56 roads. The centre of the town, known as "The Diamond", is a hub for music, poetic and cultural gatherings in the area.

There is evidence for settlements around the town dating back to prehistoric times including the remains of round forts and other earth works.

It is generally accepted by historians that St. Patrick was captured by raiders from the Clans governed by Niall of the Nine Hostages and that this region is that to which St. Patrick returned; being familiar with the people, language, customs and lands. The first Clan to convert to Christianity as the result of St Patrick's efforts was the Clan Connaill (also known at one time as Clan Dalaigh: in English, this is pronounced Daley and it translates as "one in a leadership role"). Connaill was a son of Niall of the Nine Hostages. As a result on their acceptance of Christianity, St Patrick bless the clan members and the sign of the Cross appeared on the chieftains shield and this became not only the coat of arms for the clan but also for County Donegal (up until the early 1970s).