|Primary inflows||Dunkellin river|
|Surface area||257 ha (640 acres)|
A turlough is a karst lake, which has no surface outlet and is surrounded on all sides by rising land. At Rahasane the drainage has been modified in that since the nineteenth century the Dunkellin river has followed an artificial channel downstream of the turlough, but part of the flow continues to go underground. via a natural sink, into the underlying limestone. Rahasane consists of two basins which are connected at times of flood but separated as the waters decline. It covers 257 ha at an altitude of 10–30m above sea level. The site comprises marshes, seasonally flooded wet meadows, with limestone outcrops and scrub at its margins. In the summer the lake empties and the basin is grazed by cattle, horses and sheep.
The southern basin is the more impressive feature, with high rocky sides above an undulating base, strewn with boulders. There is a low hill on the south side of the main basin, and another on the north-east, near Shanbally Castle.
It has been designated a Special Protection Area and a Special Area of Conservation. It is an important location for migrating birds, and wintering ground for the white-fronted goose. It is one of the few known breeding grounds in Ireland of the Eurasian wigeon. It is a very good place to spot accidental visitors from America, such as the American wigeon, and from Europe, such as the black tern.
The fairy shrimp Tanymastix stagnalis was first recorded in Ireland from the southern basin at Rahasane. It cannot occur in permanent waterbodies as it needs isolation from predators in order to grow to reproductive age.