Lough Derg , historic Lough Dergart (Irish: Loch Deirgeirt ),  is a lake in the Shannon River Basin, Ireland. It is the second largest lake in Ireland, and the third largest on the island of Ireland (after Lough Neagh and Lough Corrib).
It is a long, narrow lake, with beaches in counties Clare (south-west), Galway (north-west) and Tipperary (to the east). It is the southernmost of the three major lakes on the River Shannon; the other ärLough Ree and Lough Allen.Towns and villages on Lough Derg include Portumna, Kill & Ballina, Dromineer, Terryglass, Mount and Garry Kennedy.
Sea’s name evolved from the Irish Loch Deirgdheirc .  It was one of the names of the Dagda, an Irish god, and literally means “red eye”. 
At its deepest, the lake is 36 meters  deep and covers an area of 130 square kilometers (50.2 sq miles). Close downstream of where Lough Derg empties into the Shannon are the falls of Doonass, the biggest decline on the otherwise gently sloping river. Nearby is the site of hydroelectric plant in Ardnacrusha, which when built in 1927 was the world’s largest.
In the nineteenth century, Lough Derg was an important artery from the port of Limerick to Dublin through the canals in the midlands of Ireland.Navigable over its entire 40 km long, is Lough Derg today popular with cruisers and other pleasure boats, as well as sailing and fishing. The University of Limerick have an activity center by the lake, just north of Kill, where there are canoes, kayaks, windsurfing, sailing dingies and other leisure activities.
Lough Derg is home to an RNLI lifeboat based at Dromineer, the first inland station in Ireland.  In June 2013, 35 people were taken to safety when a major rescue operation was carried out by an international rowing event was hit by severe weather. [5 ]
A breeding pair of eagles first nested on an island in Lough Derg in 2012. This marked a major success for the Irish reintroduction program began in the summer of 2007.   
The northeastern shore is listed as a special area of conservation. 
Towns / Villages
- Scarriff (location of regional Waterways Ireland office) 
- List of Loughs in Ireland
- Lough Derg (Donegal), another Irish lake with the same name
- ^ Jump up to: a b placental Database for Ireland
- Jump up ^ MacKillop, James. Myths and Legends of the Celts . Penguin, 2006. p.137
- Jump up ^ International Lake Environment Committee Foundation
- Jump up ^ loughderglifeboat.com
- Jump up ^ Hilliard, Mark. “Lough Derg rescue operation gives 35 ashore by rowing event”. Www.irishtimes.com. The Irish Times. Retrieved 22 June 2013.
- Jump up ^ “Sea eagles return to the Irish nest”. The Irish Times.04.30.2012. Pulled 04/30/2012.
- Jump up ^ RTÉ: Rare eagle was reintroduced to Ireland
- Jump up ^ Havsörn
- Jump up ^ http://www.npws.ie/sites/default/files/protected-sites/conservation_objectives/CO004058.pdf
- Jump up ^ Waterways Ireland ePortal