Lough Mask (Irish: Loch Measca) is a limestone lake of about 83 km2 (32 sq mi) in Counties Mayo and Galway, Ireland, north of Lough Corrib.
Lough Mask is the middle of the three lakes, which empty into the Corrib River, through Galway, into Galway Bay.
Lough Carra flows into Lough Mask, which discharges through the Cong Canal and underground passages in the limestone bedrock of the district. The flows from the underground passages and the Cong Canal come together at the village of Cong to form the River Cong which flows into Lough Corrib.
Lough Mask is the fourth largest lake, by area, in Ireland and the sixth largest lake in the island of Ireland.
The eastern half of Lough Mask is shallow and contains many islands. The other half (Upper Lough Mask) is much deeper, sinking to a long trench with depths in excess of 50 metres.
Lough Mask has a mean depth of 15 m (49 ft), and a maximum depth of 58 m (190 ft).
Its water volume of 1.3 km3 (1.1 million acre⋅ft) is the largest in the Republic of Ireland and the second largest on the island of Ireland (after Lough Neagh).
