https://en.wikipedia.org/wiki/Fáelán_mac_Colmáin

Faelan is first mentioned in the annals when he defeated and slew Crundmáel Bolg Luatha of the Uí Cheinnselaig at the Battle of Duma Aichir in 628.[3] The annals seem to give him the title of king of Leinster by anticipation.[2] Then in alliance with Faílbe Flann mac Áedo Duib , the king of Munster and Conall Guthbinn mac Suibni of the Clann Cholmáin he defeated and slew Crimthann mac Áedo , the Uí Máil king of Leinster, at the Battle of Áth Goan in western Liffey. From this date he becomes king of Leinster.[4] The Book of Leinster gives him a reign of 30 years and his death obit of 666 in the annals appears to be an interpolation based on the king lists. It is more probable that he died before 656 when Crundmáel Erbuilc mac Rónáin is called king of Leinster at his death obit.[2]