Loingsech (d. 704), son of Óengus and king of Tara, belonged to the Uí Néill dynasty of Cenél Conaill. He is stated to have been the only son of Óengus son of Domnall (qv) son of Áed (qv), but his mother's name is not recorded. According to at least one tradition, Loingsech was married to Muirenn, a daughter of the powerful Leinster king Cellach Cualann (qv); she was the mother of his son, Flaithbertach (qv). It appears that they eventually divorced and that she subsequently married Írgalach (qv) son or grandson of Conaing of Síl nÁedo Sláine. It also emerges that Loingsech had four other sons: Artgal, Connachtach, Flann Gerg, and Fergal. Loingsech is one of only seven kings before the time of Brian Bórama (qv) to be styled rí Érenn (king of Ireland) by the original hand of the Annals of Ulster, which seems to imply that he exceeded what was normally expected of a king of Tara, achieving, perhaps, more than overlordship of the Uí Néill dynasties.
Loingsech's career, however, appears rather more prosaic in so far as it may be discerned from the record. A suspicion arises, therefore, that his acclaim as rí Érenn might have owed more to his relationship with Adomnán (qv) – he was a distant cousin and exact contemporary of the abbot – at a time when the chronicle of Iona was being compiled. Loingsech is first noticed in the record in 672 when, as a young man, he defeated and slew Dúngal son of Máel-tuile, the local ruler of Cenél Bogaine (south Co. Donegal), in an encounter at Tulach Ard. Whether or not he attained the status of king at this early stage is not clear. Certainly, Loingsech gained advantage from the internecine strife that overtook rival Uí Néill dynasties; protracted infighting within Cenél nÉogain followed the slaying of Máel-dúin son of Máel-fithrich in 681, and the killing of Fínshnechtae Fledach (qv) in 695 likewise fuelled dissension within Síl nÁedo Sláine.
It was at this point that Loingsech emerged as king of Tara, or overking of the Uí Néill. In the spring of 697, Loingsech presided at the synod of Birr and lent his full support to the Law of Adomnán. His name heads the list of kings who subscribed to the law; here too he is styled rí Érenn, although the styling may not be contemporary. For three years from 700 onwards, Ireland was afflicted by a severe famine and plague, a situation which perhaps prompted Loingsech to seek expansion into Connacht. In any event, his south-westward thrust in 704 led to his undoing. He was heavily defeated at Corann (Co. Sligo) by Cellach (qv) son of Rogallach, king of Connacht. It seems that the battle became a rout; the casualties included Loingsech himself and three of his sons: Artgal, Connachtach, and Flann Gerg.
Although the kingship of Tara passed to a cousin, Congal Cennmagair (qv) son of Fergus Fánat, one of Loingsech's surviving sons, Fergal, was prominent within Cenél Conaill. In 707 he was fighting in support of his Cenél nÉogain namesake, Fergal (qv) son of Máel-dúin. Subsequently, the kingship of Tara fell to Loingsech's son Flaithbertach, from whom the later rulers of the dynasty descended.