Fínsnechta belonged to the southern Síl nÁedo Sláine sept of the Uí Néill and was King of Brega, in modern County Meath, Ireland.
[4] He appears to have faced resistance from the King of Leinster, and a Battle of Loch Gabor (Lagore) was fought in 677.
[8] 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.
As a result, Saint Adomnan grew angry with the high king and cursed saying that his descendants would have no fame.
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 (died 701) of the Uí Chernaig sept of south Brega defeated Congalach mac Conaing Cuirre (died 696) of north Brega at the Battle of Imlech Pich in 688.
The earliest surviving list of High Kings of Ireland, that in the Baile Chuind Chétchathaig was apparently compiled in Fínsnechta's reign.