In the ‘Additamenta’ (§16) in the Book of Armagh it is stated that Áed had gone to Armagh during the abbacy of Ségéne (661–88), and gave a bequest of his church and ‘kin’ to Patrick, which means that he incorporated them in the paruchia Patricii. It is likely that Muirchú wrote his Life of Patrick soon after Fland Feblae succeeded to the abbacy following the death of Ségéne in 688 and while Áed was still at Armagh; Áed appears to have died in 700, probably in retirement as an anchorite. The date of completion would therefore probably be in the early 690s. Both Muirchú and Áed appear on the guarantor list of the Law of Adomnán (qv), drawn up at a synod of the ecclesiastics and nobles of Ireland held at Birr in 697, which suggests that Muirchú was then a person of considerable importance.
Muirchú claims Cogitosus, already the author of a Latin Life of St Brigit (qv), as his ‘father’, meaning presumably his predecessor, and possibly his mentor, in the new genre of Irish hagiography. He states that he was drawing on both tradition and written sources; the written sources probably included Patrick's Confession, a copy of which features in the Book of Armagh (written c.707). The Life is modelled on earlier hagiography, especially the Life of St Martin of Tours by Sulpicius Severus, a copy of which also features in the Book of Armagh. The purpose of the Life appears to have been to establish Armagh's entitlement to primacy in the Irish church and to exalt her over her rivals, particularly Kildare. It was also hoped that her claims on various churches could be legitimised by retrospectively associating them with Patrick, for which reason it exaggerated his role in the conversion of the Irish. Among the churches targeted by Armagh were those of Auxilius (qv) and Iserninus (qv) in the midlands and east (‘Vita Patricii’, i. 19, 3–4, p. 92), two of the earliest foundations in the country. The Life claims that the two saints were conferred with lower orders on the day Patrick was created bishop and were made his assistants.
Muirchú was probably the father of ‘Colmán son of Murchú’ (Colmán mac Murchon), abbot of Movilla (Co. Down) and author of a Latin hymn to St Michael (‘In Trinitate spes mea’). Muirchú's death is not recorded in the annals. The note in the Martyrology of Óengus (p. 145) for his feast-day on 8 June says ‘Medron and Murchon, two brothers, in Cell Murchon among the Huí Ailella. Medron and Murchu, sons of Húa Machthéni . . .'. Parts of Muirchú's Life of Patrick are preserved in three manuscripts, the most important exemplar being that in the Book of Armagh, which was compiled in the time of Abbot Torbach (qv) (d. 808).