The river is formed from three rivers and a number of streams draining off the northern slopes of Cuilcagh mountain in County Cavan, which combine underground in the Marble Arch Cave system. On the surface, the River Cladagh emerges from one of the largest karst resurgences in the British Isles, and flows through the village of Swanlinbar, before crossing the border into County Fermanagh in Northern Ireland. It then passes Kinawley before entering Upper Lough Erne and becoming a tributary of the River Erne.