Máel Sechnaill mac Maíl Ruanaid

Máel Sechnaill mac Máele Ruanaid (died 27 November 862) was High King of Ireland. The Annals of Ulster use the Old Irish title rí hÉrenn uile, that is "king of all Ireland", when reporting his death, distinguishing Máel Sechnaill from the usual Kings of Tara who are only called High Kings of Ireland in late sources such as the Annals of the Four Masters or Geoffrey Keating's Foras Feasa ar Éirinn.

Máel Sechnaill was a grandson of Donnchad Midi mac Domnaill of Clann Cholmáin, who was King of Tara from around 778 to 797. Clann Cholmáin was a sept of the Uí Néill which ruled as Kings of Mide in east central Ireland. While the southern Uí Néill had been dominated by the Síl nÁedo Sláine Kings of Brega in the 7th and early 8th centuries, the Clann Cholmáin were dominant from the time of Máel Sechnaill's great-grandfather Domnall Midi. The Kingship of Tara, a largely symbolic title, alternated between Clann Cholmáin as representatives of the southern Uí Néill and the Cenél nEógain as representatives of the northern Uí Néill.

Máel Sechnaill became king of Mide and head of Clann Cholmáin after killing his brother Flann in 845, and king of Tara in 846 on the death of Niall Caille mac Áeda of the Cenél nEógain, who drowned in the Callan River close to Armagh. He had appeared in the Irish annals some years earlier, being noticed in 839, and again 841 as a result of fighting among the chiefs of Clann Cholmáin when he killed his cousin Diarmait, son of Conchobar mac Donnchada, when Diarmait had tried to depose Máel Sechnaill's father as king of Mide.

...
Wikipedia