However, in 749 Cathussach was killed at Ráith Beithech (Rathveagh, modern County Antrim) probably in the interest of the Dál Fiatach and Fiachnae's brother, Bressal, became king.
Fiachnae defeated them at the Battle of Emain Macha, near Armagh, and Dúngal and his ally Donn Bó mac Con Brettan, king of Fir Rois were slain.
[5] In 761 Fiachnae defeated the Uí Echach Cobo (a branch of the Dál nAraidi) of the west part of county Down in the Battle of Áth Duma where their king Ailill mac Feidlimid was slain.
Donnchad may have been seeking to settle affairs involving the Uí Echach Coba and Airthir and the border region of the Conailli Muirtheimne (in modern County Louth).
[14] Fiachnae's nephew Diarmait Ua Áedo Róin (died 825) was a Céli Dé (Culdee) church reformer and founded the monastery of Diseart Diarmad (Castledermot, modern County Kildare) in 812 renewing the ties of Bangor with Leinster.