Éogan (d. 465), son of Niall, is the eponymous ancestor of the royal lineages of Cenél nÉogain. Éogan and his brothers Conall Gulban (qv) and Énnae have perhaps the best historical claim to represent the sons of Niall Noígiallach (qv), although the genealogies accord them eleven half-brothers including Conall Cremthainne (qv), Cairpre (qv), and Lóegaire (qv). The ‘Banshenchas’ (lore of women) tradition that names the mother of Éogan and Conall Gulban as Indiu, daughter of Óengus Finn, is possibly the earliest – although an alternative version gives Rígnach, daughter of Meda son of Ross. Either way, it seems there was a recollection of early marriage alliances between the earliest Uí Néill rulers and the Ulster dynasty of Dál Fiatach. There is no record of Éogan's wife or wives, but he is credited with ten or eleven sons, the most prominent being Muiredach, from whom the kings of Cenél nÉogain descended, and Eochu Binnech, ancestor of Cenél mBinnich.
Uí Néill historical tradition celebrates Éogan (along with his brother Conall Gulban) as a conqueror of the north-west, but it seems reasonable that this achievement should be credited not to him personally but to his heirs. The favourable treatment of Éogan in later Patrician hagiography reflects the close relationship of his descendants with Armagh; when the sons of Amolngid are barred from Tara, it is to ‘his faithful friend Éogan’ that St Patrick (qv) makes appeal. Subsequently, the king and his family receive the saint's blessing.
Éogan's death is placed by the annalists under the year 465. His immediate progeny do not feature very prominently, but by the sixth century Cenél nÉogain constituted a dynasty of some account in the north-west, their powerbase being the peninsula of Inis nÉogain (Inishowen). Some sixth-century descendants of Éogan's son Muiredach, including Muirchertach/Mac Ercae (qv), are said to have been kings of Tara, but these claims are problematic. There is more reason to consider that Cenél Conaill held a primacy at this early stage; this would accord with the tradition of ‘Timna Néill’ (the testimony of Niall), which has Éogan receive ‘military might’ from his father, while Conall Gulban receives the sovereignty.