As related in various hagiographical accounts, Éogan was captured by British raiders while still a youth. Rescued from slavery through the intervention of St Ninian (qv), he became a clerical student at the latter's foundation of Candida Casa at Whithorn in Galloway. Another version of events has Éogan abducted to Brittany. He is said to have shared these adventures with two young Irishmen who later became church founders: Tigernach (qv) (d. c.549) of Clones and Cairpre of Coleraine. Quite possibly, these stories of capture are duplicated and are a device designed to account for a British influence in the foundations ascribed to Éogan and to some of his contemporaries.
It is claimed that Éogan made his first foundation in Leinster at Cell na Manach (Kilnamanagh, Co. Dublin) and that, with the assistance of the saintly monks Lóchán and Énnae, he kept a school there. His students are said to have included Cóemgen and, according to a medieval Life by John of Tynemouth, the British saint Petroc. It is held that during his sojourn in Leinster, Cainnech (qv) of Achad Bó, with other saints, helped him to combat demons. Certainly, the cult of Éogan became well established in Leinster and is reflected in dedications. Cell nÉogain in Mag nAilbe (south Co. Kildare) is a case in point; this was the seat of Cenél Nath Í, a minor lineage which traced its ancestry to Dál Messin Corb. Other sites perhaps included the now extinct placename Kelloghane (north Co. Kildare), and Clonogan (Co. Carlow).
According to his Life, Éogan moved northwards and became associated with Ard Sratha (Ardstraw, Co. Tyrone), a church whose first bishop, Mac Ercae, had allegedly been consecrated by St Patrick (qv). Éogan is one of several early ecclesiastics with alleged Dál Messin Corb connections commemorated in mid-to-east Ulster – but whether such links, in any given case, represent personal migration or cult diffusion is uncertain. The date of Éogan's death is assigned in the martyrologies to 23 August, but the year is not recorded. It is suggested in Acta SS Hib. that he flourished c.570, which is not improbable. Of Éogan's churches, the record suggests that Kilnamanagh had fallen out of use before the twelfth century. Ardstraw was recognised as a diocesan see at the synod of Ráith Bressail (1111), but the bishop's residence was moved firstly to Maghera and then to Derry.