Niall (d. 1063), son of Eochaid and overking of Ulaid, belonged to the dynasty of Dál Fiatach. His father, Eochaid (d. 1004), had reigned as overking; his mother was Máel-miadach, daughter of a certain Ua Flathra. He had at least one older half-brother, Dub-tuinne (dubbed In Torc, ‘the boar’), and a sister, Ben-Ulad. The name of Niall's wife is not recorded, but he had at least two sons, Eochaid and Donnchad. His father was slain in 1004 at Cráeb Tulcha (Crew Hill, parish of Glenavy, Co. Antrim), along with several of his kinsmen and many nobles of the Ulaid, in battle against the Cenél nÉogain king Áed Ua Néill (qv), who also fell. This catastrophe initiated a time of strife within Dál Fiatach: the overkingship passed in rapid succession to two of Niall's uncles, to a distant cousin, to his brother Dub-tuinne (d. 1007), and to two of the latter's sons, Domnall (d. 1007) and Niall (d. 1016), with each contender slain in turn and his place taken by rivals.
Niall son of Eochaid first came to prominence in 1012, when still a very young man. After the plundering of Ulaid for the second time by the Cenél nÉogain king Flaithbertach Ua Néill (qv), he rebelled against his nephew, Niall son of Dub-tuinne and defeated him in the battle of Na Mullaig. According to the annals, Niall son of Eochaid was accepted as king shortly afterwards. His nephew, however, unwilling to abandon his claims to the royal dignity, drew support from neighbouring Dál nAraide (technically a sub-kingdom of Ulaid) and forced him to battle in 1016. Niall was victorious, and secured his position as overking. The record shows him to have been one of the most durable holders of that dignity. He reigned for forty-seven years and did much to restore the stability of a province wracked by dynastic conflict.
In the early stages, Niall faced both internal and external threats to his position, which he met with equal resolve and ruthlessness. In 1020 he seized and blinded a cousin, Flaithbertach grandson of Eochaid, to suppress opposition within the dynasty. In 1022 he defeated a naval fleet from Dublin and, having slain a great number of the Hiberno-Scandinavians, imprisoned many of the survivors. He also sought to expand the influence of Dál Fiatach beyond the boundaries of Ulaid; that same year he moved against the neighbouring kingdom of Airgialla and defeated its forces at Sliab Fuait (in the Fews, Co. Armagh). He then took the offensive against the Hiberno-Scandinavians, plundering their territory in 1026 and carrying away much booty. Doubtless this action provoked Flaithbertach Ua Néill, king of Cenél nÉogain, then engaged in establishing his own supremacy over Mide and Dublin; the following year he raided Ulaid and took a cattle-tribute. This did not deter Niall from boldly mounting an expedition against Cenél nÉogain in 1031. He reached their royal centre of Telach Óc (Tullaghogue, Co. Tyrone) but, apparently, his initiative failed and many of his men and cattle were captured in a counter-attack. Closer to home, a punitive raid on the territory of Uí Echdach (west Co. Down) met with greater success.
The following decade was a time of decidedly mixed fortunes for Niall. Perhaps the plunder of Uí Echdach by Cenél nÉogain in 1041 had no great impact on him, but in 1045 the Dublin Norsemen attacked Rathlin Island and slaughtered a party of Ulstermen, including a kinsman, Ragnall grandson of Eochaid. Later the same year Niall joined with the king of Mide, Máel-Sechlainn Ua Máelshechlainn, to plunder Fingal and burn Swords. Seemingly, by this time he had formed an alliance with the powerful overking of Leinster, Diarmait (qv) son of Donnchad Máel na mBó (qv). When severe famine struck Ulster in 1047, Niall and many of his retainers fled south to Leinster, but how long they remained there is not certain. In 1048, and again the following year, he accompanied Diarmait on campaigns in Mide; he was active in Osraige in 1052. It is not clear whether these operations were conducted from a base in Ulster, or whether he remained exiled in Leinster throughout this time. In the latter event, it must be assumed that his immediate family maintained control of the kingship on his behalf; neither the king-lists nor the annals admit another ruler. There is no suggestion, however, that he played any active part when the Cenél nÉogain raided the Ulaid sub-kingdom of Dál nAraide on two occasions in the late 1050s. His son Eochaid, styled rígdamna Ulad (eligible for the kingship of Ulaid), died in 1062.
Niall, explicitly described as airdrí Ulad (high-king of Ulaid), died on Thursday 13 November 1063, presumably aged in his early-to-mid seventies. Unfortunately for Dál Fiatach, the death of Niall signalled the re-emergence of conflict within the dynasty. The overkingship was claimed in turn by three dynasts of uncertain lineage, at least two of whom were probably distant cousins, before Niall's grandson Donn Slébe son of Eochaid emerged to secure the dignity in 1071. Although two subsequent Dál Fiatach overkings belonged to a lineage that adopted the surname of Ua Mathgamna, most of the later kings were descended from Niall's son Eochaid or from his grandson Donn Slébe. Their lines became known by the surnames of Ua hEochada (O'Hoey) and Mac Duinnshlébe (McDonleavy).