(6th century), abbot, founder of Penmon church (Anglesey) and its dependency Ynys Seiriol, now called Puffin Island. He and his friend Cybi are Anglesey's principal saints. His well and ruined beehive cell survive at Penmon, as does a monastic settlement on nearby Puffin Island. Like many other Celtic saints he probably travelled by sea and settled near the coast on low-lying land, living first as a hermit and founding a community later. His well would have been used both for his own water supply and for baptism. The later monastic buildings at Penmon are a suitable memorial: they were inhabited by Austin Canons who frequently revived forgotten shrines. Bede mentions Edwin of Northumbria's capture of Anglesey from Cadwallon, the Welsh king, but does not state that Edwin besieged him in Ynys Seiriol in 632. Matthew Arnold in the poem ‘East and West’ repeats Celtic legends of Cybi and Seiriol, who lived in West and East Anglesey respectively. The only known dedications to Seiriol are at Penmon and Puffin Island: it seems that he was a saint of purely local cult. Feast: 1 February.
From The Oxford Dictionary of Saints in Oxford Reference.