small island, with an area of 0.7 square mile (1.8 square km), off the tip of the Lleyn Peninsula, Gwynedd county, historic county of Caernavonshire (Sir Gaernarfon), Wales. It is separated from the mainland by a channel 2 miles (3 km) wide that has a strong tidal race. On this naturally protected site was the first religious house in Wales, founded by the Celtic St. Cadfan in the early 6th century; it was superseded by an Augustinian abbey. A place of pilgrimage in the Middle Ages and later a haunt of pirates, Bardsey Island is now a major British nature reserve and bird sanctuary.