(6th century), abbot, founder of a monastery at Holyhead (called in Welsh Caer Gybi, ‘Cybi's Fort’) in Anglesey, the small island on which it stands being called Holy Island or Ynys Gybi. With his friend Seiriol he is generally regarded as the most important saint of Anglesey. There are, however, other traces of Cybi in Welsh and Cornish place-names. Llangybi, near Pwllhelli (Gwynedd), had a well and half-ruined ‘beehive’ cell still called St Cybi's Well which could date back to his time. This is situated in a valley on the edge of a wood, about a quarter of a mile from the parish church, which has in its graveyard a very ancient carved cross. The well, used for baptism and healing, is a most impressive early Christian monument. There is another Ynys Gybi, an islet off the north coast of Dyfed, and another Llangybi in the valley of the Teifi river.
A dedication to Cybi at Llangibby-on-Usk, together with Cornish dedications at Cuby, Tregony, and Landulph, have helped to establish the view of Cybi as a Cornish saint who travelled by sea and river, living as a hermit and/or evangelist in the various places which bear his name and finally settling on Anglesey, where his most important work was achieved. This is much less fanciful than the 13th-century Life which makes him go to Jerusalem, stay with St Hilary (d. 449) for fifty years and be consecrated by him as bishop, and then go to Ireland via Menevia for several years before arriving in Anglesey. The claim, however, that he was given the site of his monastery there by the Welsh prince Maelgwn Gwynedd of Cunedda may well be correct. It is an interesting, almost unique Welsh example of a church being situated in the enclosure of a Roman fort, presumably a coastguard station. Feast in Cornwall and Wales generally on 8 November; other dates in Wales 5, 6, 7 November, but in Cornwall (Roscarrock) 13 August.
From The Oxford Dictionary of Saints in Oxford Reference.