Áed Findliath (‘light-grey’) (d. 879), son of Niall and king of Tara, belonged to the Northern Uí Néill dynasty of Cenél nÉogain. His father, Niall Caille (qv), previously king of Tara, died in 846. His mother, Gormlaith, was a daughter of Donnchad Midi (qv) son of Domnall Midi (qv) of the Southern Uí Néill dynasty of Clann Cholmáin. He had five brothers or half-brothers, the most prominent of whom was Flaithbertach, and at least one sister, unnamed in surviving sources, who married Conaing king of Brega. Áed was married three times: to Gormlaith Rapach, daughter of Muiredach son of Eochaid king of Ulaid, mother of his son Domnall; to Máel-Muire, daughter of Cináed (qv) son of Alpín king of Scots and Picts, mother of his son Niall Glúndub (qv); and to Lann, sister of Cerball (qv) son of Dúngal, king of Osraige, widow of Máel-Sechnaill (qv) son of Máel-ruanaid of Clann Cholmáin, king of Tara, and the mother of his son Flann Sinna (qv). It is not clear which of Áed's wives was the mother of his daughter Eithne, who married Flannacán king of Brega, to whom she bore a son, Máel-mithig.
Áed first came to notice in 855 when he led a raid on the Ulaid which, although it inflicted some damage on his traditional enemies, cost the life of his brother Flaithbertach. The following year, he defeated a Norse–Irish force at Glenn Foichle (Glenelly, Co. Tyrone). Next in seniority to Máel-Sechnaill, the powerful king of Tara, Áed Findliath allied himself with the vikings of Dublin in an effort to ensure that the paramount kingship should not be monopolised by a rival dynasty; he married off one of his daughters to Amlaíb Find (Olaf the White), king of Dublin (Frag. Ann., §292). He also allied himself with his sister's son Flann, king of Brega, whose brother (or half-brother) Cináed (qv) son of Conaing had been drowned by Máel-Sechnaill. Challenging the king of Tara, Áed and his allies launched minor raids on Mide (861, 862). Máel-Sechnaill had invaded the north in 860, on which occasion he had to repel an assault that Áed launched on his camp during the night.
Áed's marriage to Lann, of the Osraige, assumed greater significance when he attained the kingship of Tara (862/3). Cerball, now the second most powerful king in Ireland, was allowed a free hand in Munster. Not all of Áed's alliances, however, stood the test of time. In 868 his nephew Flann son of Conaing rebelled against him with the support of the Laigin and at least some Norse forces. Áed defeated this consortium at Cell ua nDaigre (in Co. Meath) and slew Flann, but his cousin Fachtna son of Máel-dúin was also among the fallen. The Laigin were later made to pay: in 870 Áed over-ran Leinster as far as Gabrán, while his brother-in-law Cerball launched a parallel invasion from Osraige. He again invaded Leinster in 874, profaning Cell Ausili (Killashee, Co. Kildare) and burning other churches. The Ulaid, likewise, were held under Áed's authority: in 871 he instigated the slaying of Cathalán son of Indrechtach, a king of the Ulaid, probably for attempting to repudiate his overlordship. Meanwhile, Áed steadfastly opposed attempts by the vikings of Strangford Lough to increase their presence in northern Ireland: in 866 he rooted out several of their settlements on the Ulster coast, killing 240 of them in one battle near Lough Foyle. Yet he remained loyal to his allies the Dublin Norsemen. He took firm action in 875, when Dublin was seized by Albann (qv) from Northumbria; he helped to drive off the latter and supported Amlaíb's kinsman, Barid (qv).
Áed also had a reputation for piety; it is perhaps appropriate that a later writer should have attributed to him the view that ‘battles are not won by numerousness of warriors but through the help of the Creator and through the righteousness of the king’. He had close connections with Armagh, and had a house there (a man was slain in it in 870). A feature of the later years of Áed's reign was his apparent unwillingness to hold the Óenach Tailten (fair of Tailtiu; Teltown, Co. Meath): failure to hold the óenach, without what the annalist considered to be good reason, is noted at 873, 876, and 878. Áed died on 20 November 879 at Druim Inasclainn in Conaille (Dromiskin, Co. Louth), and is commemorated by a poem in the Annals of Ulster. His immediate successor in the kingship of Cenél nÉogain was his first cousin Murchad (d. 887) son of Máel-dúin, although Domnall (d. 915) son of Áed emerged from 887 to displace Murchad's son, Flaithbertach. Throughout this time, the kingship of Tara was held by Flann Sinna son of Máel-Sechnaill, till 916, when Áed's son Niall Glúndub succeeded to this dignity. Most of the later rulers of Cenél nÉogain descended from Áed, although at least one, Áed grandson of Ualgarg (d. 1067), descended from his brother Flaithbertach.