Cuilcagh is a mountain on the border between County Fermanagh (in Northern Ireland) and County Cavan (in the Republic of Ireland) (once part of the a medieval Kingdom of Breifne). With a height of it is the highest point in both counties and the 165th highest on the island of Ireland. Water from the southern slope flows underground until it emerges some miles away in the Shannon Pot, the traditional source of the River Shannon.