Niall Glúndub (d. 919), son of Áed and king of Tara, belonged to the Northern Uí Néill dynasty of Cenél nÉogain. His father was the powerful Áed Findliath (qv), who died as king of Tara in 879. His mother was Máel-Muire (d. 913), a daughter of Cináed (qv) son of Ailpín, king of the Scots and Picts. He had a half-brother Domnall (d. 915), and a sister named Eithne who married Flannacán son of Cellach king of Brega, and was the mother of his son Máel-mithig (d. 919). It seems that Niall married Aillinn, daughter of Ainbith son of Áed king of Ulaid, by whom he had his most illustrious son, Muirchertach na Cochall Craicinn (qv); an alternative tradition, however, gives Lann, daughter of Eochaid king of Dál Riata, as the mother of Muirchertach. Later he married Gormlaith (qv), daughter of Flann Sinna (qv), the Clann Cholmáin king of Tara; it is claimed that she had earlier been the wife of Cormac (qv) son of Cuilennán and of Cerball (qv) son of Muirecán, overkings respectively of Munster and Leinster. It is not clear whether Gormlaith or another wife was the mother of Niall's sons Conaing (d. 937) and Máel-Ciaráin.
Niall, who acquired the sobriquet Glúndub (black-knee) for a reason that remains obscure, was still very young when his father died, so the Cenél nÉogain kingship of Ailech passed firstly to his father's cousin Murchad (d. 887), and subsequently to the latter's son Flaithbertach (d. 896). By 905 Niall had gained sufficient confidence to challenge his brother Domnall, then established in the kingship of Ailech. The brothers made terms and, for the following six years, collaborated in asserting the lordship of Cenél nÉogain over the north, in subduing Connacht, and in challenging the authority of Flann Sinna. In 908 they invaded Meath and burned Tlachtga (the Hill of Ward), a site of symbolic importance to the Southern Uí Néill. In 910 Niall plundered the north Connacht kingdom of Uí Fhiachrach and slew its ruler. Three years later he pushed deeper into Connacht and inflicted a severe defeat on the Uí Amalgada and the Umaill.
Meanwhile, when Domnall went into religious retirement in 911, Niall succeeded to the kingship of Ailech. His drowning in 912 of Cernachán son of Duiligén, a dynast of the Airthir, may relate to the claims of his dynasty over Armagh, where his father had in fact maintained a house. In June 914 he led an expedition into Ulaid in which he defeated the ruler of Dál nAraide, Loingsech grandson of Lethlobar (qv), at Fregabal (near Glarryford, Co. Antrim) and then defeated the provincial overking at Carn Éirenn (near Ballymena). Nonetheless, a contingent of Niall's troops, caught unawares, suffered heavy casualties.
Although his mother Máel-Muire had, some time after his father's death, married Flann Sinna and had become part of the Southern Uí Néill establishment, Niall was not unwilling to attempt an initiative against his step-relations. In December 914 he led a northern army into Meath, but suffered a serious reverse near Crossakeel in which a number of the Cenél nÉogain and their Airgialla allies perished. It seems, however, that relationships with the king of Tara were soon restored; it may have been about this time that Niall married Flann's daughter Gormlaith. The following year he supported the aging Flann who, in the last year of his reign, faced a rebellion by his own sons. He brought an army into Meath and forced the submission of the rebels.
Doubtless, this timely intervention greatly facilitated Niall's accession to the more prestigious kingship of Tara with the death of his father-in-law (25 May 916). In August that year he revived the important Óenach Tailten (fair of Tailtiu; Teltown, Co. Meath), which had been allowed to lapse for many years. A year later, as a clear threat had emerged from the influx of vikings under Sitriuc Cáech (qv) and Ragnall (qv) at Loch dá Cháech (Waterford Harbour) and at Cenn Fuait ‘in eastern Leinster’ (probably Glynn, near the River Barrow, parish of St Mullins, Co. Carlow). Niall led his forces southwards to support the provincial kings of Leinster and Munster, halting on 22 August 917 in Mag Femin (probably the plain of that name in south-east Co. Tipperary). After some indecisive skirmishing, Niall encamped against the vikings and sent word to the Leinstermen to draw them off by attacking their longphort (fortified camp) at Cenn Fuait; but the Leinster army was destroyed and the vikings reoccupied Dublin. During the following year he maintained sporadic warfare against Sitriuc Cáech.
In the autumn of 919, with the support of the Ulaid and the Airgialla, Niall marched against Dublin, but on 14 September was slain and his forces completely routed near the settlement – apparently at Cell Moshamóc, adjacent to the ford at Islandbridge. Other fatal casualties included his nephews Máel-mithig king of Brega, and Flaithbertach son of Domnall. Niall's last battle has a prominent place in literature. In view of his close involvement with poets and poetry – he is alleged to have been a patron of Cormacán Éces – it is fitting that his death is the subject of several works of verse, including one attributed to his wife Gormlaith. Although Niall's son Muirchertach was king at a later date, in the aftermath of the battle of Dublin the kingship of Ailech passed to his nephew Fergal (d. 938) son of Domnall, and that of Tara to Donnchad Donn (qv) son of Flann.