Lough Mask, lake, in Counties Mayo and Galway, Ireland. It covers an area of 32 sq miles (83 sq km) and is connected with Lough Corrib by an underground river. Lough Mask is noted for its brown trout. The island of Inishmaan in Lough Mask contains the remains of a Celtic church of St. Cormac, built in the 6th century and enlarged in the 12th century.