Lough Derg or Loch Derg is a lake in County Donegal, Republic of Ireland. It is near the border with Northern Ireland and lies about 7 kilometres (4.3 mi) north of the border village of Pettigo. It is best known for St Patrick's Purgatory, a site of pilgrimage on Station Island in the lake. St. Patrick is said to have established the first Christian settlement here on Saint's Island. The pilgrimage was moved to Station Island in 1497.