Ua Néill, Flaithbertach (d. 1036), son of Muirchertach and king of Ailech, belonged to the Cenél nÉogain dynasty of the Northern Uí Néill. In later sources he is sometimes referred to as Flaithbertach an Trostáin (of the pilgrim's staff). His father Muirchertach (slain 977), styled rígdamna Érenn (eligible for the high-kingship of Ireland), was a son in turn of Domnall Ua Néill (qv) (d. 980), king of Tara. His mother was Cres Cumal, who belonged to the Uí Maine of eastern Connacht. He himself married Bébinn, daughter of Brian Bórama (qv), who was the mother of his sons Domnall (d. 1027) and Áed (d. 1033). He had at least two other sons, Muiredach (slain 1039) and Niall.
Flaithbertach succeeded to the kingship of Ailech in 1004, when his uncle Áed Ua Néill (qv) son of Domnall was slain at the battle of Cráeb Tulcha (Crew Hill, parish of Glenavy, Co. Antrim). He dedicated the earlier years of his reign to subduing the Ulaid, before tackling rival Uí Néill dynasties including Cenél Conaill (in the northwest) and midlands-based Clann Cholmáin, whose ruler Máel-Sechnaill (qv) son of Domnall, although now overshadowed by Brian Bórama, was still overking of Uí Néill. In 1005 Flaithbertach invaded Ulaid from the south, defeating and slaying the local kings of Leth Cathail and Uí Echdach (baronies of Lecale and Iveagh, Co. Down) in two separate battles. The following year he experienced a serious setback on an incursion into Conaille Muirthemne (north Co. Louth), when Máel-Sechnaill, who perhaps already viewed him as a threat, intervened and defeated him. Nonetheless, in 1007 he subdued Leth Cathail for the second time and took hostages. Two years later he again turned his attention to Cenél Conaill: he seized and blinded a local ruler of Inis Éogain (Inishowen), before leading a foray into the plain of Brega – part of Máel-Sechnaill ‘s realm.
In the summer of 1010 Flaithbertach made formal submission at Armagh to his father-in-law Brian Bórama and yielded hostages. The following year, he joined with Murchad son of Brian to plunder Cenél Conaill taking herds of cattle and many captives. Shortly afterwards, he invaded Dál Fiatach – the heartland of Ulaid – and destroyed the fortress of Dún Echdach. In 1012 he was again campaigning in Cenél Conaill and led his forces down through the Barnesmore Gap as far as Drumcliff. During his absence Máel-Sechnaill struck at Cenél nÉogain, but seemingly to little effect. Undaunted, Flaithbertach once more invaded Ulaid, pillaging the Ards peninsula, and took great spoils. When the kings of Leinster and of Dublin, Máel-mórda (qv) son of Murchad and Sitriuc Silkbeard (qv) son of Amlaíb (Óláfr) Cuarán (qv), revolted against Máel-Sechnaill and Brian in 1013, Flaithbertach hosted as far as Kells, where a stand-off took place.
After the battle of Clontarf he continued to harass Máel-Sechnaill, now the leading claimant to high-kingship. In 1017 he slew a local ruler of Uí Meic Uais in Meath, and the following year made another foray against Clann Cholmáin. Later he plundered Cenél Conaill, and in 1021 his cousin Domnall son of Áed took spoils from Óenach Macha (adjacent to Armagh) despite the vigorous opposition of several Airgialla kings. He took advantage of the intra-dynastic strife in Meath which followed the death of Máel-Sechnaill (1022): in the summer of 1025 he led an army through Brega and as far as Fine Gall, taking the hostages of the Irish from the men of Dublin. Early the following spring, he crossed frozen rivers to sack the fortress of Inis Mochta in Meath.
In the closing years of his reign, Flaithbertach undertook expeditions against the Ulaid (1027) and Cenél Conaill (1028), both of which resulted in great spoils and tributes – it is not clear, however, that he was playing as active a role as before. In 1030 he abdicated the kingship and went on pilgrimage to Rome, returning the following year. He died in 1036, after ‘perfect penitence’, according to his obit. His immediate successor, his son Áed, in whose favour he had abdicated, predeceased him on 30 November 1033. Flaithbertach's other son, Muiredach, then succeeded, but he was slain in 1039. The kingship at this point passed to the distantly related Niall (qv) (d. 1061) son of Máel-Sechlainn from whose brother Lochlainn the family line of Mac Lochlainn of Cenél nÉogain seems to have descended.