The Fergus River springs have the largest catchment (115 sq km) of any of the Burren resurgences, the drainage area reaching almost to Galway Bay in the north. The catchment has been delimited with reasonable accuracy using water tracing and computation of specific runoff values. The drainage system feeding to the Fergus springs, including the Fergus River Cave, may be of considerable antiquity and there is some evidence that part of the drainage to St. Brendan’s Well at Lisdoonvarna is being captured by the Fergus system.