Ua Néill, Domnall (d. 980), grandson of Niall Glúndub (qv) and king of Tara, belonged to the Northern Uí Néill dynasty of Cenél nÉogain, the cause of which had been greatly advanced by his immediate forebears. He had at least three brothers, Flaithbertach (slain 949), Murchad, and Flann; and a sister, Dúnlaith. The Uí Néill overkingship had become destabilised as a result of the death of Domnall's father Muirchertach na Cochall Craicinn (qv) in 943, and the death of Muirchertach's Clann Cholmáin rival Donnchad Donn (qv) son of Flann Sinna (qv) the following year. The vacuum impelled Ruaidrí Ua Canannáin (qv) and Congalach Cnogba (qv) son of Máel-mithig to come forward from the long-suppressed dynasties of Cenél Conaill and Síl nÁedo Sláine and claim the kingship of Tara.
An opportunity arose for Domnall, however, when Ruaidrí, who had previously slain Flaithbertach Ua Néill, fell in battle against the Norse of Dublin (950). Turning his attention to Congalach, who still held the kingship of Tara, he secured the support of Dublin and ravaged Mide (954). Congalach's death two years later at the hands of the Norsemen left the way clear for Domnall to seize the paramount kingship. The regnal list accords him a reign of twenty-five years as king of Tara. Having achieved supremacy among the Uí Néill, Domnall aimed to assert his authority over the other provincial kings. His marriage to Echrad, who belonged to the royal lineage of Osraige, should perhaps be viewed as a diplomatic stratagem towards this end. She became the mother of his son Muirchertach. Domnall had at least two other sons, Muiredach and Áed Ua Néill (qv) (d. 1004). By 962 he was at war with Munster and engaged in reasserting his authority over Mide. Three years later, he took the hostages of Fergal (qv) grandson of Ruarc, then overking of Connacht. In 968 he ravaged Leinster and the Norse kingdom of Dublin.
Significantly, Domnall's main struggles were not with the Norsemen, who by this time were already in decline as a military power within Ireland, but rather with the Leinstermen and the men of Mide. Clann Cholmáin proved a constant thorn in his side, in spite of the fact that his sister Dúnlaith was married into that dynasty and was the mother of Máel-Sechnaill (qv) son of Domnall son of Donnchad Donn. A protracted campaign against Clann Cholmáin (970–71) resulted in Domnall's expulsion from Mide, causing an annalist to lament the deposition of a rightful king and to prophesy widespread crop-failure as punishment. In the event, he was back the following year, whereupon he adopted the strategy of placing a garrison in Mide. This innovation effectively ended the struggle; the strategy was especially notable in that it anticipated the more territorial notion of overlordship that developed in the course of the following two centuries.
It is surely significant that Domnall is one of only eight Uí Néill rulers to be styled rí Érenn (king of Ireland) in the original hand of the Annals of Ulster, while his obit at 980 represents the first contemporary usage of the title ard-rí Érenn (high-king). His son Muirchertach had been slain by the king of Dublin in 977 and, ironically, his immediate family did not manage to emulate his achievements. While his son Áed (qv) and grandson Flaithbertach Ua Néill (qv) reigned as kings of Ailech, primacy within the Cenél nÉogain dynasty passed to the rival line of Mac Lochlainn; it was not till the thirteenth century that the Ua Néill line managed to regain its former dominance. Domnall's immediate successor as king of Tara, and the claimant to a developing high-kingship of Ireland, was his nephew and rival, Máel-Sechnaill of Clann Cholmáin.