Áed Allán (d. 743), son of Fergal and king of Tara, belonged to the Cenél nÉogain dynasty. His father, Fergal (qv) son of Máel-dúin, king of Tara, was slain by the Leinstermen in 722. His mother was probably a daughter of Ernán, a Cenél Conaill dynast, although the Fragmentary Annals relate a dramatic tale of Áed's conception after a secret tryst between Fergal and a nun, a daughter of Congal Cennmagair (qv), king of Tara. In any event, Áed had three half-brothers: Conchobar, Colcu, and the pious Niall Frossach (qv), who later held the kingship of Tara from 763 to c.770, when he abdicated to join the church.
Áed came to prominence from 732 onwards when he clashed relentlessly with the forces of the then king of Tara, Flaithbertach (qv) son of Loingsech (qv), of the rival Cenél Conaill dynasty. The final conflict between them took place at sea (734), with a battle fought at the mouth of the River Bann; Flaithbertach's fleet and that of his allies, the Dál Riata, were defeated by the Cenél nÉogain. Áed's victory not only forced the abdication of Flaithbertach, but led to the exclusion of Cenél Conaill from the political mainstream, so that the kingship of Tara alternated between the dynasties of Cenél nÉogain and Clann Cholmáin for centuries. Moreover, the plain of Mag nÍtha (the Finn valley) was alienated to Cenél nÉogain.
Áed's ambition to gain access to the east coast brought him into conflict with the Ulaid, and in 735 he won an important victory at the battle of Fochairt (Faughart, Co. Louth). The overking of Ulaid, Áed Rón (qv), and one of his sub-kings were slain. Áed Allán's victory was so complete that it led to an interregnum in the kingship of the Ulaid. According to the Annals of the Four Masters, this battle was precipitated by the profanation of the church of Cell Chonna, which was under the protection of Armagh. On Tuesday 19 August 738 Áed inflicted another crushing defeat on his enemies, the Leinstermen, at Áth Senaig (possibly Ballyshannon near Kilcullen, Co. Kildare), where he avenged the killing of his father Fergal sixteen years earlier. According to the annals, Áed fought the Leinster king, Áed Menn (qv) son of Colcu, in hand-to-hand combat, and was wounded before he decapitated his opponent with a sword. No fewer than thirteen Leinster nobles were slain in this battle.
In 737 Áed had held a rígdál (royal meeting) with the king of Munster, Cathal (qv) son of Finguine, at the neutral location of Terryglass, Co. Tipperary. It is thought that a non-aggression pact came about as a result of the meeting: Cathal agreeing to let Áed have a free hand in Leinster, Áed making it clear that as a strong and ambitious king of Tara he would not tolerate any more raids on the Southern Uí Néill. It is also thought that Cathal accepted the supremacy of Armagh at this meeting, the annals stating that the Law of Patrick (qv) was now recognised throughout Ireland. A very successful overking, Áed had not only managed to eliminate his great rivals the Cenél Conaill from the kingship of Tara, but had inflicted crushing defeats on his enemies the Ulaid and the Laigin. His power was even felt in Munster, their king preferring to agree to a diplomatic division of power.
Áed Allán was slain (743) at the battle of Seredmag (near Kells, Co. Meath), which he fought with the king of Mide, Domnall Midi (qv) son of Murchad Midi (qv), who attained the kingship of Tara. Áed's son Máel-dúin (d. 788), though not king of Tara, reigned as king of Ind Fhochlae – ‘the north’ – which suggests that he dominated the Northern Uí Néill. Nonetheless, his descendants were the relatively unimportant Muintir Birn and Muintir Eruilb. The later kings and overkings of the dynasty of Cenél nÉogain descended from Áed's brother, Niall Frossach. A great deal of apocryphal material concerning Áed has survived in Irish folklore. Only a pseudo-explanation is available for the sobriquet Allán.