Niall Caille (d. 846), son of Áed Oirdnide and king of Tara, was a member of the Northern Uí Néill dynasty of Cenél nÉogain. His father, Áed Oirdnide (qv) son of Niall Frossach (qv) king of Tara, died in 819. According to the ‘Banshenchas’ (lore of women), his mother was Medb daughter of Indrechtach who, if he is to be identified with the son of Muiredach Menn, died in 752. He had four brothers or half-brothers, including Máel-dúin (d. 867), Fogartach, Blathmac, and Máel-Calainn. Niall may well have contracted more than one marriage, but Gormlaith (d. 861) daughter of Donnchad Midi (qv) was his queen. He had at least six sons – Áed Finnliath (qv), Flaithbertach (d. 855), Muirchertach, Dub-indrecht, Óengus, and Bróen – and an unnamed daughter who married Conaing king of Brega and was the mother of his son Flann.
Niall emerged as ruler of Ailech from 823, in succession to his second-cousin Murchad son of Máel-dúin (d. 788), whom he deposed. It appears, however, that Murchad retained local political importance within Cenél nÉogain and later supplied military aid to his overlord. Clearly a major force in the north by 827, Niall supported the cause of Éogan Mainistrech (qv), his anamchara (spiritual adviser), in a contest for the headship of Armagh; towards this end, he fought a successful battle at Leth Cam, near the ecclesiastical centre. This victory brought the Airgialla firmly under Niall's control, and greatly increased his influence at Armagh. In 833, on the death of Conchobar (qv) son of Donnchad Midi, he was strong enough to take the kingship of Tara.
In the course of the following thirteen years Niall managed to thwart efforts by the vikings to establish themselves on a firm basis in the north; he asserted the authority of the Northern Uí Néill over the midlands and Leinster, and engaged in a contest for political supremacy with the powerful Éoganacht overking of Munster, Fedelmid (qv) son of Crimthann. Shortly after his accession to the kingship of Tara, Niall, with the support of his cousin Murchad, defeated a viking force that had attempted to plunder Derry. In 835 he invaded Leinster, which he brought under his sway in no uncertain terms. He deposed the Leinster overking, Cellach son of Bran Ardchenn (qv), replacing him with an Uí Dúnchada rival, Bran (qv) son of Fáelán, from whom he exacted a tribute. Returning northwards, he plundered Meath to enforce his authority on that province.
Niall recognised that open exercise of Uí Néill lordship within Leth Moga (the southern half of Ireland) clashed directly with the aspirations of the Éoganacht overking, who in the previous decade had intervened in the midlands with some success. The rígdál (royal meeting) between Niall and Fedelmid son of Crimthann at Cloncurry, Co. Kildare, in 838 was doubtless an attempt to redefine their respective spheres of influence; in all probability, the claim of the Annals of Inisfallen that Niall submitted to his rival represents Munster propaganda. In any event, the agreement reached was short-lived. In 840 Niall was drawn back into conflict when Fedelmid invaded Meath and encamped at Tara. Fedelmid retreated before the advancing Uí Néill army, but not before he had plundered Meath's border-territories and, at least according to Munster accounts, captured Niall's queen Gormlaith and slain his nephew, Indrechtach son of Máel-dúin.
The following year Niall was again obliged to respond when Fedelmid advanced into Leinster to celebrate the Óenach Carmain (Fair of Carman, east of Kilcullen, Co. Kildare) – an assertion of sovereignty over the Laigin. The Cenél nÉogain caught the Éoganacht forces off guard at Mag nÓchtair (Co. Kildare), inflicting a defeat that resulted in the collapse of Munster's challenge to Uí Néill supremacy. The intervening years, however, had witnessed an increased level of viking activity in Ireland, with the emergence of permanent settlements. Niall continued to hold the Vikings at bay, in 843 defeating an invading force that had made its way up Lough Swilly into the valley of the River Finn (Co. Donegal).
Three years later Niall was drowned near Armagh, apparently by accident, in the River Calann (Callan) – hence his posthumous sobriquet Caille. His immediate successor in the kingship of Ailech was his brother Máel-dúin, but his son Áed Finnliath subsequently acceded to the kingships of Ailech and Tara. His son Muirchertach is probably to be identified with the abbot of Derry who died in 882. Niall Caille, who features in a number of bardic poems of later date, is the ancestor through his grandsons Domnall and Niall Glúndub (qv) of most of the later Cenél nÉogain kings.