County Fermanagh is the westernmost of the six counties that form Northern Ireland, and the westernmost part of the United Kingdom. Fermanagh is often referred to as Ireland's Lake District. The county is approximately 120 km (75 mi) from Belfast and 160 km (99 mi) from Dublin. Fermanagh was a stronghold of the Maguire clan before the confiscation of lands relating to Hugh Maguire.

The name of the county is derived from the tribe called Fir-Monach. Monach who was fifth in descent from Cahirmore, King of Ireland from A.D. 120 to 123, settled on the coast of Lough Erne towards the end of the third century, and his descendants eventually spread themselves over the entire county. For several centuries the county belonged to the family of Maguire, and hence was known as the "Maguire's Country." The ancient name of Enniskillen was "Inis-Cethlenn," called after Kethlenda, who was the wife of the famous mythical hero "Balor of the Mighty Blows," chief of the sea robbers called Formorians. Bellisle was formerly called Ballymacmanus or Senat Macmanus, after the family of Macmanus, its owners. Cahal Maguire, Dean of Clogher lived there in the 15th century, one of the greatest of Irish Scholars and Historians: his "Annals of Ulster" is one of the most valuable works dealing with Irish history. "Tooraw" was the ancient name of that part of the county which lies between Lough Melvin and Lough Erne. The Baronies of Clankelly and Clanawley are named after the tribes which inhabited these districts.

County Fermanagh is a predominantly rural region with its geography dominated by two lakes: Upper Lough Erne and Lower Lough Erne. This 70-km (40 mi) stretch of lakes, known collectively as Lough Erne, is now linked to the River Shannon and its waterways via the Shannon-Erne Waterway canal, making the entire system the longest navigable inland waterway in Europe. Smaller lakes in the county include Lough Scolborn. In addition, Fermanagh shares many lakes: Lough Melvin (with the Republic of Ireland), Lough Macnean Upper and Lough Macnean Lower (both lying on the Cavan border).

The peak of Cuilcagh, part of the Cuilcagh Mountains range, on the Fermanagh/Cavan border is the highest point in Fermanagh (665 metres, 2,182 feet). Other mountains/ranges include Belmore Mountain, Tappghan Mountain (on the border with Tyrone), Brougher Mountain (on the border with Tyrone), Molly Mountain, Benaughlin Mountain, Slieve Rushen (on the border with Cavan), Derrin Mountain, and Slieve Beagh (on the Tyrone border). Much of the western and eastern areas of the county are forested.