Conall Cremthainne (d. 480?), an early dynast of Uí Néill and putatively king of Uisnech, was regarded by the pre-Norman genealogists as the common ancestor of Síl nÁedo Sláine and Clann Cholmáin. Conall (also called Conall Err Breg) is included among the fourteen sons credited to Niall Noígiallach (qv). These dynastic ancestral figures, including Éogan (qv), Lóegaire (qv), Cairpre (qv), and Conall Gulban (qv) – the latter perhaps identical with Conall Cremthainne – are represented as brothers, the chronological difficulties of such a relationship notwithstanding. Tradition seems uncertain whether the mother of the brothers Conall was Indiu daughter of Lugaid, or Rígnach daughter of Meda; in either case a link with the Ulster dynasty of Dál Fiatach is suggested. Other northern connections may be reflected in the sobriquet accorded to this Conall, which the tract ‘Cóir Anmann’ associates with Uí Chremthainn, an Airgialla dynasty. Alternatively, his name stems from the territory of Cremthenn in Mide (between Slane and Knowth, Co. Meath), and it is possible that his identity was merged with that of an earlier north-midland figure around the time that the dynastic federation of Uí Néill was formed. Some tenth- and eleventh-century Clann Cholmáin dynasts were styled ‘king of Cremthenn’.
The development of Conall's persona is difficult to trace with confidence, but indications are that he was acquiring a divided identity by the later seventh century, as Patrician and Brigidine hagiography took shape. It is not unlikely, as several recent commentators have argued, that the Conall episodes in the ‘Collectanea’ of Tírechán (qv) (in which the dynast grants a church-site to St Patrick (qv) and is acclaimed as the ancestor of kings), and in the ‘Vita Prima’ of St Brigit (qv) (in which he is spared from the murderous intent of his brother Cairpre), have Conall Cremthainne in mind – if only because Síl nÁedo Sláine interests underlie these productions. Even so, it seems that Conall's identity was formulated piecemeal over time. It is significant that the hagiographers treat Conall as a single personage. Likewise, the so-called ‘Timna Néill’ (the testimony of Niall), has the patriarch bequeath the ‘primacy’ to Crimthann, while his ‘sovereignty’ is left to Conall.
In all likelihood, the division of Conall's identity resulted from dynastic shift, as the lineages of Síl nÁedo Sláine and Clann Cholmáin, which emerged supreme in the midlands in the seventh century, increasingly sought to dissociate themselves from their Cenél Conaill kindred. Conall Cremthainne is accorded an obit at 480 in a proto-historic stratum of the Annals of Ulster. His inclusion in the regnal list as king of Uisnech is almost certainly an anachronism. Genealogical doctrine (probably dating to the eighth century) claims him as the father of Ardgal, whose death in the battle of Detnae (in Co. Meath), at the hands of a ruler of the Airthir, is placed at 520. Another alleged son of Conall Cremthainne was Fergus Cerrbél (probably a compound character), whose son (or grandson) Diarmait (qv) son of Cerball reigned as king of Tara.