Lough Derg (Loch Deirgeirt in Irish) is the largest lake (or lough) in the Republic of Ireland and the second largest in Ireland overall. It borders parts of counties Galway, Clare, and Tipperary. The lake is the last of three on the River Shannon, with the other two, Lough Ree and Lough Allen being further north. Some towns or villages on Lough Derg include Garrykennedy , Portumna , Killaloe & Ballina, Dromineer and Terryglass .

At it's deepest, the lake is 36 metres and covers and area of 118sq km. An island in Lough Derg is of religious significance. It is called Saint Patrick's Purgatory because Saint Patrick is reputed to have stayed there in 445.

The lake is a popular place for leisure boating and fishing. The University of Limerick have their activity centre by the lake, just outside Killaloe. It is used for canoes, kayaks, windsurfing and yachts, amongst other purposes.

See also: List of Irish lochs and loughs