Lough Derg, historically Lough Dergart, is a freshwater lake in the Shannon River Basin, Ireland. It is the third-biggest on the island of Ireland (after Lough Neagh and Lough Corrib) and the second largest lake in the Republic of Ireland.
It is a long, narrow lake, with shores in counties Clare (south-west), Galway (north-west), and Tipperary (to the east).
It is the southernmost of three large lakes on the River Shannon; the others being Lough Ree and Lough Allen. Towns and villages on Lough Derg include Portumna, Killaloe & Ballina, Dromineer, Terryglass, Mountshannon and Garrykennedy.
