Domnall (d. 642), son of Áed and king of Tara, belonged to the dynasty of Cenél Conaill. His father was Áed (qv) son of Ainmere (qv), king of Tara; his mother was Lann daughter of Áed Guaire, a king of the Airgialla. He had at least one brother, Máel-Cobo. According to Adomnán (qv), who was distantly related to Domnall, the latter had been present as a child at the synod of Druim Cett (near Limavady, Co. Londonderry; 575) and had been blessed by Colum Cille (qv). Although his brother Máel-Cobo had laid claim to Uí Néill overkingship for three years prior to his death in 615, Domnall's immediate predecessor in that position was Suibne Menn (qv) of the rival lineage of Cenél nÉogain. Domnall, who emerged to challenge the latter in the 620s, was married to Dúinsech (d. 638), whose origins are obscure. It is not stated that she was the mother of Domnall's five sons, who are named as Óengus (slain 650), Fergus Fánat, Ailill, and Conall and Colggu (both slain 663).
Historical literature represents Domnall as a heroic warrior and as ‘king of Ireland’; the reality is certainly impressive. Initially, Domnall struggled to attain kingship and was fortunate to escape with his life from an encounter with Suibne Menn at Both (perhaps Raphoe, Co. Donegal) in 628. Conveniently for Domnall, Suibne was slain that same year by Congal Clóen (qv), Dál nAraide overking of Ulaid. It is tempting to suggest collusion, but in view of later developments it may be more reasonable to interpret the killing as coincidence. Domnall was now paramount king of the Uí Néill; the threat of Cenél nÉogain had been temporarily laid to rest and no representative of the dynasties in the midlands was ready to mount a meaningful challenge. Significantly, Domnall is featured in ‘Baile Chuinn’, the Old Irish list of kings of Tara, in addition to Middle Irish lists. In these, he is accorded a reign of thirty years, which is difficult to reconcile with annalistic data.
To demonstrate his new status, Domnall made an incursion into Leinster in 628, it being almost mandatory for Uí Néill overlords to assert their authority over Leth Moga (the southern half of Ireland). On this occasion, however, there was little response from the Leinstermen, preoccupied as they were with their own internal dynastic conflicts. In 629 he was ready to move against Congal Clóen and, with the support of at least one of the Airgialla kings, he defeated him at Dún Cethirn. This achieved, he was supreme in the north and could count on the support of his nephew Conall Cóel (qv), son of Máel-Cobo. When it emerged in 637 that Congal Clóen had secured an alliance with Cenél nÉogain and with Dál Riata, Domnall organised a concerted move against his adversaries. While his nephew Conall Cóel defeated the combined enemy fleet of Sailtír (Kintyre), Domnall shattered the Ulaid forces and those of the Dál Riata king, Domnall Brecc (qv), at Mag Roth (Moira, Co. Down).
This battle was significant not only because it marked the effective end of Ulaid ambitions to control the north, but because it signalled the emergence of Airgialla as a mesne kingdom within the Uí Néill sphere of influence. This may be viewed as Domnall's real achievement. He died in 642, peacefully it seems, and significantly is styled rex Hiberniae (king of Ireland) in his obit. His immediate successors were his nephews Conall Cóel and Cellach, who dominated the Uí Néill realms in the north-west, but do not seem to have exercised similar authority in the midlands. Domnall's own sons were not especially prominent, but his grandsons Loingsech (qv) and Congal Cennmagair (qv) later reigned as kings of Tara.