Lough Mask is a large limestone lake straddling Counties Mayo and Galway in the west of Ireland, covering approximately 32 square miles (83 sq km), roughly 20,000 acres in extent. It runs about 10 miles long by 4 miles wide and sits approximately 30 km north-west of Galway city and 20 km south-east of Ballina, accessible via the N84 and R310 roads through Cong and Clonbur.

The lake is notable for its extensive reef systems lying just beneath the surface — a serious hazard for any vessel. These submerged limestone reefs are capable of causing severe hull damage and demand careful navigation, particularly for paddlers unfamiliar with the water. The lake contains numerous islands, and the middle section between Carrigeendauv in the north and Ram's Island in the south is characterised by deeper water and complex island-edge topography.

Access points are distributed around the lough. Cushlough Bay on the eastern shore is the primary access point, offering a large car park, an anglers' shelter, and public toilets. Other access points include Cahir Bay, Ballinchalla Bay, Roshill, Ferry Bridge, Annagh, Churchfield, Tourmakeady, Srah, Ballygarry, and Aughinish. Local permission is required at some of these locations. Cather Pier, approximately 5 km from Ballinrobe, is also used as a launch point. Access to the lake itself is free, with no admission charges.

The lake's character varies considerably across its extent. Cushlough Bay is shallow with heavy weed growth in summer. The western side of the lake is generally deeper than the eastern side. The Bay of Islands area features varied currents around island edges, while Ram's Horn Bay holds deeper water. The lake is connected to Lough Corrib by an underground river.

The island of Inishmaan within the lough contains the remains of a Celtic church of St. Cormac, originally built in the 6th century and enlarged in the 12th century, providing a point of historical interest for those exploring by water.

Wildlife on and around the lake includes brown trout, ferox trout, European perch, and northern pike. Public transport to the area is limited, with regional bus services stopping in Cong and Ballinrobe, and taxi connections available for onward travel to lakeside access points.