Despite his legacy as author of a text as famous as the Félire, little is certain about Óengus’ career. The Martyrology of Tallaght, a text also potentially written by Óengus, has a later addition which records the day of his death as March 11, and identifies him as a bishop. The exact year of his death is not known, but if the information from the Prose Preface is correct, he was a contemporary of the Uí Néill king Áed Oirdnide (AU 797–819), the canon-lawyer Fothud na Canóine (AU 819.1), and Connmach, abbot of Armagh (AU 807.1). He is said to have attended a synod headed by Áed Oirdnide at Dún Cuair in 804 AD at which Fothud freed clergy from hosting—an event documented in  the Annals of Ulster (AU 804.7-8).

He appears to have been a member of the community of Clonenagh before joining the community of Tallaght. According the poem Aibind suide sund amne, Clonenagh is where he learned his first psalms and where he found his final resting place. The Prose Preface relates that, when he moved to Tallaght, he tried to hide his status and learning and took up work in the kiln drying corn. He was finally discovered through a miracle: a little boy had miraculously learned all his lessons after sleeping on Óengus’s knee.

In the Félire, however, we get a glimpse of how he perceived himself: he describes himself as a pauperán and deidblén, meaning ‘pauper’ in the sense of a cleric, as well as a céle Chríst ‘a servant of Christ’. His membership of the community of Máel Rúain of Tallaght has led him to be associated with the céle Dé or Culdees of Tallaght. They have long been considered a reform movement, but modern scholarship suggests the community is perhaps better regarded as a strict monastic community emphasizing the ascetic lifestyle, devotion, and pastoral care.

Some of the manuscripts insert Óengus’s genealogy into the Prose Preface or, as in the Leabhar Breac, into the Prologue. If correct, this genealogy places his descent somewhere in Co. Down, possibly in the vicinity of Warrenpoint.

On f. 63v of manuscript Laud Misc. 610, Óengus’s own feastday has been added to the entry for March 11th in the Félire, together with a gloss offering a substitute couplet for the original. It reads: