A Turlough is a lake that appears at the wettest time of the year, winter, and disappears again in the late spring. Turloughs are found in karst limestone landscapes, and their name is usually taken to be from the Irish tur lough meaning “dry lake”.
In the summertime they are grasslands, usually grazed by cattle.This is the case at Dún na Sí as, in times past, the area where the Turlough is now formed part of what was called the ‘Cow Park’, a commonage where locals allowed their cattle to graze freely.
In some turloughs, there is a permanent pond or wetland in the middle. In the wintertime, when the water table rises, turloughs turn into lakes. They fill quickly – over a matter of hours or days – via hollow, water-filled galleries in the bedrock that open out into one or more swallow-holes. These can be hard to spot when the turlough is dry, and it's unclear where all the swallow-holes of the Dún na Sí turlough are.
The type of underground water system that causes the Turlough to appear and disappear, according to the seasons, is virtually unique to Ireland. Because of their unusual environmental conditions, turloughs support very special ecosystems rich in plants and insects.