King Cadwallon ap Cadfan of Gwynedd is said to have sheltered here in around 630 when fleeing an invasion from the Kingdom of Northumbria. A monastery existed on the island in the late 12th century and was mentioned by Gerald of Wales, who visited the area in 1188. He claimed that, whenever there was strife within the community of monks, a plague of mice would devour all their food. Llywelyn the Great issued two charters in 1221 and 1237 confirming the Canons Regular, in possession of the island and the church and manor of Penmon on the mainland of Anglesey.