He became King of Brega and High King in 675, after killing his predecessor, and first cousin, Cenn Fáelad in battle at Aircheltra, a place which is not identified. The Fragmentary Annals of Ireland relate how Finsnechta won support by his generosity. Among those he won over were the king of Fir Rois and Saint Adomnán. According to this annal, he had been granted a stewardship by Cenn Fáelad, but was unsatisfied with this and encouraged by his friend of Fir Rois he challenged Cenn Faeladto battle and won the kingship. He appears to have faced resistance from the King of Leinster, a Battle of Loch Gabor was fought in 677. There was mutual slaughter on both sides but Finsnechta emerged the victor. In 680, Finsnechta had the King of Leinster, Fiannamail mac Máele Tuile, assassinated. Finsnechta was involved in the north as well. In 676, he destroyed Ailech, center of Cenél nEógain power. He fought Bécc Bairrche mac Blathmaic, King of Ulster, in 679 at the Battle of Tailtiu. In June 684, Ecgfrith, the King of Northumbria, sent an army under Berht to Ireland which laid waste to the plain of Brega, the heart of Fínsnechta's kingdom. The reasons for the sending of the army are unclear. The hostages taken in the raid were later returned through the offices of Adomnán in 687. According to the saga Bóroma, it is mentioned that the Fínsnechta Fledach undertook an expedition against Leinster when the Laigin refused to pay the cattle tribute. The Leinster king Bran Mut mac Conaill assembled the Leinster forces and sent Saint Moling, the abbot of Ferns, to negotiate with Fínsnechta. Mo-Ling tricked Fínsnechta into remitting the tribute. As a result, Saint Adomnan grew angry with the high king and cursed saying that his descendants would have no fame. Fínsnechta did penance to Adomnan and was forgiven for remitting the Boruma.
Temporary Abdication and Death
In 688, Fínsnechta abdicated to become a monk, but he left the clerical life and resumed the kingship in 689. This return to power may have been a result of the break-out of a civil war among the Síl nÁedo Sláine when Niall mac Cernaig Sotal of the Uí Chernaig sept of south Brega defeated Congalach mac Conaing Cuirre of north Brega at the Battle of Imlech Pich in 688. Also, Áed mac Dlúthaig of the Síl nDlúthaig sept killed Diarmait Dian mac Airmetaig Cáech, the King of Uisnech, of the Clann Cholmáin in 689. He was killed, along with his son Bresal, by his kinsmen Áed mac Dlúthaig and Congalach mac Conaing Cuirre at Grellaigh Dollaith. According to the Annals of Tigernach, this occurred in battle, but the Fragmentary Annals of Ireland state that Finsnechta was murdered in a tent by Congalach. Congalach succeeded Fínsnechta as King of Brega, and Loingsech mac Óengusso, of the northern Cenél Conaill branch of the Uí Néill, was the next High King. Fínsnechta's descendants the Clan Fínsnechtai played no important role after him. Fínsnechta was survived by a son named Ailill who was killed in 718. The earliest surviving list ofHigh Kings of Ireland, that in the Baile Chuind Chétchathaig was apparently compiled in Fínsnechta's reign.
