The Aille River rises in the Partry Mountains 16 km (10 mi) south of Westport, drains the northwest part of the Partry range and sinks underground at the Aille Caves. It continues as a subterranean river for 4 km (2.5 mi), resurging at Pollatoomary. It flows south to Cloon Lough, which feeds into Lough Mask.