Small island at the mouth of the River Neb at Peel, enclosed by the walls of Peel Castle.

St Patrick's Isle stands immediately off Peel and is now joined to the town by a causeway. Its position at the entrance to the harbour made it both a religious centre and a fortified site.

The island became the seat of the cathedral church of St German. An early account states that Bishop Simon, who died in 1247, was buried in St Patrick's Isle in the Church of St German, which he had begun to build.

The medieval fortress developed around the cathedral and other ecclesiastical buildings. Later descriptions regarded the castle on St Patrick's Isle as exceptionally strong, one account calling it impregnable except by famine.

The island's name carried the tradition of St Patrick's connection with Mann. Medieval and later writers told of Patrick reaching the Island during his missionary journeys, although the surviving textual traditions vary considerably in date and detail.

The enclosed site contains the remains of churches, domestic buildings and fortifications from different periods. It continued to serve as a military post and prison after the cathedral had ceased to function as the centre of the diocese.