It is 32,000 acres in size and joins three counties of Clare, Galway and Tipperary.
There is a monastic ruin with a round tower on the aptly named Holy Island.
This settlement was found in the 7th century and trips to the island are available from Mountshannon.
It is the third-largest lake in Ireland It is a long, relatively narrow lake.
The lake is the last of three on the River Shannon, with the other two, Lough Ree and Lough Allen lying further north.
The area around Lough Derg is considered to be affluent and the towns and villages reflect that.
Killaloe and Ballina, Dromineer, Terryglass and Mountshannon are pretty towns that thrive on the amenities that the lake offers.
Navigable over its full 40 km length, Lough Derg is today popular with cruisers and other pleasure traffic.
Kayaking, canoeing and sailing schools operate on the lake throughout the year and Lough Derg is a big earner for tourism in the mid west.
At its deepest, the lake is 36 metres deep and covers an area of 118 km2.
The lake is a popular place for leisure boating, sailing and fishing.
At the point where Lough Derg empties into the Shannon, it slopes steeply downhilla major reason for the location of the world's then-largest hydro-electric plant at Ardnacrusha in 1927.
