Lough Mask, Lake in County Mayo, Ireland

Lough Mask is a large freshwater lake in County Mayo, in the west of Ireland, running about 10 miles (16 km) in length. Its limestone bed gives the water a light tone, and its shoreline alternates between rocky outcrops and open grassy edges with small islands scattered across the surface.

In 1338 Sir Edmond de Burgh was drowned in the lake by his cousin Sir Edmond Albanach Bourke at the end of the Burke Civil War; he was captured at Ballinrobe and killed on Oilean-an-lara (Earls Island). On 1 April 1651 the scribe Dáibhídh Bacach Ó Duibhgeannáin was working on Oileán Ruadh (Red Island) in the house of Tadhg Og O Flaherty. In 1882 the Lough Mask Murders saw a grandfather and grandson serving as bailiffs killed by tenant farmers, their bodies dumped in the lake.

The lake is known for its trout, and the World Cup Trout Fly Fishing Championship is held each year at Cushlough Bay near Ballinrobe. Local legend gives Bly Island a banshee, said to haunt it and sometimes heard along the shore. The Irish name is Loch Measca, used as a pen-name by Séan Seoighe in a 1929 Irish-language memoir.

The lake is reachable by road from several directions in County Mayo, and boat rentals are available nearby for those who want to get out on the water. Spring and early summer tend to offer the most settled weather for a visit.

Lough Mask and the neighboring Lough Corrib are connected not by a visible river but through an underground network of limestone channels. Water moves between the two lakes beneath the surface, which makes their shared boundary invisible to anyone standing on the shore.