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 and the second largest lake in the Republic of Ireland.

The Shannon River Basin consists of the area containing Ireland's longest river, the River Shannon, and all of its tributaries and lakes. The official Ordnance Survey Ireland length of the Shannon from its Shannon Pot source is 224 miles (360 km) made up of 63.5 miles (102.2 km) tidal water flow and 160.5 miles (258.3 km) freshwater flow.