There is little doubt that Fiacc later became a bishop – the first to be consecrated in Leinster – and Patrick himself is supposed to have presented him, after his consecration, with the insignia of his episcopal office. Before entering religion he had been married and had one son, Fiachra, who afterwards became a bishop, was buried at Sletty with his father, and was venerated there. The location of Fiacc's first church foundation, named Domnach Féicc in the sources, is now forgotten, but it was probably in the territory of the Uí Bairrche, in the present-day parish of Moyacomb, near Shillelagh on the Carlow–Wicklow border. Alternatively, it may be identical with Domnach Mór (Donaghmore, Co. Wexford), the site of a very early church foundation. His last and most famous foundation was at Sletty, where he established a primitive monastic community of seven companions and where, because of the sanctity and austerity of his life, he acquired the honorific title ardepscop (archbishop), which in those early times had no connotations of metropolitan status.
Muirchú (qv), the earliest biographer of Patrick, writing towards the end of the seventh century, portrays Fiacc as a student of Dubthach of the moccu Lugair, ‘who afterwards became an excellent bishop, whose relics are worshipped in Sléibte’ (§I 19 (18)). In the ‘Additamenta’ in the Book of Armagh, he is described as ‘Fiacc the Fair . . . the first bishop who was consecrated in Leinster’ (§13, 3–5). Fiacc is also mentioned in the martyrologies and genealogies, including the Echternacht copy of the Martyrologium Hieronymianum. He is erroneously credited with the authorship of an Old Irish metrical Life of Patrick, ‘Génair Patraicc’ (probably of eighth-century composition and written at Sletty) and of a Latin poem in honour of St Brigit (qv), ‘Christus in nostra insula’. His feast-day is given as 12 October.