Saint Seiriol was a sixth-century Welsh monk and hermit who founded a monastery at Penmon on the island of Anglesey (Ynys Mon) in North Wales and later withdrew to the small island off its coast that came to bear his name. He belonged to the generation of Celtic monastic founders who established religious communities across Wales during the Age of the Saints, and he is remembered chiefly as a hermit who drew disciples to a settled monastic life before retiring once more into solitude.
According to tradition, Seiriol was of royal descent, a younger son of the ruling house of Rhos in North Wales. His brother Einion, who ruled the region of Llyn, granted him a dwelling on the southeastern tip of Anglesey, where Seiriol established his cell. There he was joined by disciples, and a monastic community grew up around him at Penmon, of which he served as the founding abbot. In his later years he retired to the neighbouring island of Ynys Lannog, afterwards called Ynys Seiriol, where he had founded a daughter house of the Penmon monastery and where he is said to have reposed.
