Dictionary of National Biography, 1885-1900/Seiriol
SEIRIOL (fl. 530), Welsh saint, was son of Owain Danwyn ab Einion Yrth ap Cunedda Wledig, according to the Hafod MS. of ‘Bonedd y Saint’ (Myvyrian Archaiology, 2nd edit. p. 415) and later authorities (Myv. Arch. p. 429; Iolo MSS. pp. 113, 125). He was therefore a cousin and contemporary of Maelgwn Gwynedd [q. v.], and probably brother to the ‘Cuneglase’ (Cynlas) of Gildas. Becoming a monk, he founded the monastery of Penmon, Anglesey, which, with the offshoot on Priestholm or Puffin Island (known in Welsh as ‘Seiriol's Isle’), continued to exist in one form or another to the Reformation. The parish church of Penmon is dedicated to Seiriol, whose festival, according to the ‘History of Anglesey’ (1775), is 1 Feb. Tradition says that Seiriol and Cybi, who founded the monastery at Holyhead, used daily to meet near two springs (still bearing their names) at Clorach, near Llanerch y Medd, and that the difference in the position of the two travellers in relation to the sun caused a difference in their respective complexions, which was commemorated by the names ‘Seiriol Wyn’ (White) and ‘Cybi Felyn’ (Tawny) (Llwyd, Beaumaris Bay, 1800). Matthew Arnold has embodied this tradition, though not quite correctly, in the sonnet beginning ‘In the bare midst of Anglesey they show.’
[Rees's Welsh Saints; authorities cited.]