Lough Derg is a freshwater lake in the Shannon River Basin.
It is the second-biggest lake in the Republic of Ireland and the third-biggest on the island of Ireland (after Lough Neagh and Lough Corrib).
It is a long, narrow lake, with shores in counties Clare, Galway and Tipperary.
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.
The lake's name evolved from the Irish Loch Deirgdheirc.
This was one of the names of The Dagda, an Irish god, and literally means "red eye".
At its deepest, the lake is 36 metres deep and covers an area of 130 km˛ (50.2 sq miles).
Reference: Wikipedia