Áed Oirdnide (d. 819), son of Niall and king of Tara, belonged to the Cenél nÉogain dynasty. His father, Niall Frossach (qv), was king of Tara until his abdication c.770. His mother Dunlaith was a daughter of Flaithbertach (qv) son of Loingsech (qv), a Cenél Conaill dynast who had held the kingship of Tara till 734. Áed had at least three brothers, including Colmán, Ferchar, and Muirchertach. While Áed's marriages are not recorded, he had at least five sons – Niall Caille (qv), Máel-dúin (d. 867), Fogartach, Blathmac, and Máel-Calainn – and a daughter, Lann, married to Cathal, king of Airgialla.
Nicknamed Ingor (the undutiful), Áed first came to prominence in 789, when he defeated a rival, Domnall son of Áed, at the battle of Clady. While holding the less prestigious kingship of Ailech, he was defeated in 791 by the then king of Tara, Donnchad Midi (qv) son of Domnall (qv) of the Clann Cholmáin. On Donnchad's death (797), however, Áed attained the paramount Uí Néill kingship, and proved to be a strong expansionist ruler in his stand against the kings of Leinster. He began his reign as king of Tara by devastating Mide, which he again invaded in 802 to restore law and order, dividing the territory between Conchobar (qv) and Ailill, sons of the late king Donnchad. Described as ‘a vigorous and adroit man of war’, Áed then invaded Leinster, twice in 804 and again in 805, when he deposed Fínshnechtae Cetharderc (qv) son of Cellach (qv), king of Leinster, who belonged to the Uí Dúnchada lineage. Áed then divided Leinster between Muiredach son of Ruaidrí and Muiredach son of Bran (qv), who respectively represented the rival Uí Fháeláin and Uí Muiredaig lineages. Áed appears to have violated the sanctuary of the monastery of Tallaght, for which his Óenach Tailten (fair of Tailtiu; Teltown, Co. Meath) was boycotted by the monastic community so that ‘neither horse nor chariot arrived there’. In recompense Áed made ample reparation to the monastery, which accepted the compensation, although the Céli Dé reformers at Tallaght had reservations about accepting gifts from sinful laity.
Áed liked to pose as a champion of the church, especially when it may have been politically advantageous to do so. In this context, in 804 he presided over a convention of the Uí Néill (congressio senatorum nepotum Néill), held at Dún Cuair (probably Rathcore, Co. Meath), where Uí Néill forces gathered to raid Leinster. It is claimed that at this convention, presided over by Condmach, abbot of Armagh, Áed was formally ‘ordained’, hence his posthumous sobriquet Oirdnide. The evidence for clerical ordination in this period is, however, scant, and the episode may be anachronistic. There are stronger grounds for accepting that in 806 Áed endorsed a renewal of the Law of Patrick (qv). Around the same time, it is thought, he also supported efforts to exempt ecclesiastics from the obligation to serve on hostings.
In 808 Áed faced a challenge when Muirgius (qv) son of Tommaltach, overking of Connacht, supported Conchobar, king of Mide, in a rebellion against him. Áed harried the borders of Mide, however, and put his enemies to flight. The following year, he turned his attentions again to Leinster and invaded that province; the Annals of Inisfallen record that he was defeated. In any event, Áed was shortly afterwards able to defeat the Ulaid and raid Ulster from the River Bann to Strangford Lough ‘in revenge for the violation of the shrine of Patrick’ – this had occurred when an Ulster nobleman, Dúnchú princeps of Tulach, was murdered in the abbot's house. In 815 the Cenél Conaill slew Áed's brother Colmán. Áed died in 819, but not before he had plundered Leinster again as far as Glendalough (817).
A marginal note in a contemporary Irish manuscript refers to Áed as rex Hiberniae (king of Ireland), although this title is unknown to Einhard, who refers vaguely to a rex Scotorum (king of the Irish) in his life of Charlemagne. Áed spent his reign in an attempt to create a greater paramountcy for the Northern Uí Néill, being one of the most powerful overkings to come from the Cenél nÉogain. He was perhaps powerful enough to discourage viking raids on Ireland for a time: a pause has been noted in the record of attacks between 814 and 820. Áed was slain at Áth dá Fherta in Mag Conaille (north Co. Louth), although the Annals of Inisfallen mistakenly place his death in Scotland; according to some accounts, his killer was one Máel-Cainnich. He was buried at Armagh. Áed was succeeded as king of Ailech by his first cousin once removed, Murchad (deposed 823) son of Máel-dúin, while the more prestigious kingship of Tara passed to Conchobar son of Donnchad Midi of Clann Cholmáin. Both dignities later reverted to Niall Caille son of Áed. Of Áed's other sons, Máel-dúin and Blathmac became the ancestors of lesser royal lines.