County Donegal (pronounced /ˈdʌnᵻɡɔːl/ or /ˌdʌnᵻˈɡɔːl/; Irish: Contae Dhún na nGall) is a county of Ireland. It is part of the Border Region of the Republic of Ireland and is in the province of Ulster. It is named after the town of Donegal (Irish: Dún na nGall, meaning "fort of the foreigners") in the south of the county. Donegal County Council is the local council for the county and Lifford serves as the county town. The population of the county is 158,755 according to the 2016 census. It has also been known as (County) Tyrconnell (Tír Chonaill), after the historic territory of the same name.

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 small 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 with a population of 19,588. Letterkenny and the nearby city of Derry form the main economic axis of the northwest of Ireland. Indeed, what became the City of Derry was officially part of County Donegal up until 1610.

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