Lough Derg(red lake), a lake in the county Donegal, Ireland, about 7 m. E. S. E. of Donegal, 3 m. long, and 2 1/2 m. wide at the broadest part. It is enclosed on all sides except the south by steep, barren hills from 700 to 1,200 ft. high. It lies 467 ft. above the level of the sea, and is 75 ft. deep. A multitude of islands dot its surface, one of which, called Station island, about an acre in extent, contains a cave known as St. Patrick's Purgatory, to which from 10,000 to 15,000 pilgrims from all parts of Ireland resort annually between June 1 and Aug. 15. They remain on the island, which contains two chapels, a house for the priests, and a few cabins, from three to nine days, their only food during that time being bread and water. Saint's isle, which was the original seat of St. Patrick's Purgatory, contains the remains of a priory founded in the 7th century. - There is another Lough Derg, forming an expansion of the river Shannon between Tipperary and Galway, about 24 m. long and from 2 to 6 m. wide.