Lough Derg (Loch Dearg in Irish, meaning "Red Lake") is a small lake in County Donegal, Ireland.

In the middle of the lough is an islet known as Station Island or, more popularly, St Patrick's Purgatory, which has been a place of pilgrimage since ancient times and remains very busy to this day. Here pilgrims fast (being allowed only one meal a day, of black tea or coffee and dry toast) and make barefoot circuits of the rocky islet during their 3-day pilgrimage.

Another Lough Derg is situated in Munster in the south of Ireland.

See also

• list of Irish lochs and loughs