His father had been slain in battle in 735 by the High King of IrelandÁed Allán of the Cenél nEógain and the kingship of Ulaid passed to the rival Dál nAraidi of southern County Antrim in the person of Cathussach mac Ailello. However, in 749 Cathussach was killed at Ráith Beithech probably in the interest of the Dál Fiatach and Fiachnae's brother, Bressal, became king. Bressal himself was killed in 750 but the Dál Fiatach retained the kingship. The historian Professor Byrne, however, believes that it is probable there was an interregnum in Ulaid between the reigns of Áed Róin and Fiachnae.
Reign
Fiachnae restored the fortunes of the Dál Fiatach during his long reign. In 759 he became involved in a dispute among the churchmen of Armagh. Fiachnae supported the abbot Fer-dá-Chrích versus a priest named Airechtach who had the support of Dúngal mac Amalgado of the Uí Néill of Brega. 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. In 761 Fiachnae defeated the Uí Echach Cobo of the west part of county Down in the Battle of Áth Duma where their king Ailill mac Feidlimid was slain. The Uí Echach Coba were to suffer another defeat in 776 this time at the hands of the Airthir. The Dál nAraidi proper engaged in internecine civil wars in 776 and 783. In one of these conflicts in 776 Fiachnae's son Eochaid gave support to the claimant Tommaltach mac Indrechtaig and they defeated and slew the ing of Dál nAraidi, Cinaed Ciarrge mac Cathussaig, and his ally Dúngal, king of the Uí Tuirtri in the Battle of Drong. The power of Fiachnae was such that the high kingDonnchad Midi sought a conference with him at Inis na Ríg in eastern Brega. However, mutual distrust prevented Fiachnae from coming ashore and Donnchad from going out to sea to meet him. The purpose of the meeting may have been to deliminate spheres of influence. Donnchad may have been seeking to settle affairs involving the Uí Echach Coba and Airthir and the border region of the Conailli Muirtheimne.
Expansion of Dál Fiatach influence
The expansion of the Dál Fiatach northwards to the shores of Lough Neagh began in his reign and this cut off the Dál nAraidi proper from their kinsmen the Uí Echach Coba in the south. Fiachnae also gave his patronage to Bangor, traditionally a Dál nAraidi monastery. He also converted Downpatrick into a royal monastery. The first recorded abbot of Downpatrick in the annals has a death notice in 753. His sons Eochaid mac Fiachnai and Cairell mac Fiachnai were Kings of Ulster. Another son, Loingsech mac Fiachnai was abbot of Downpatrick. Fiachnae's nephew Diarmait Ua Áedo Róin was a Céli Dé church reformer and founded the monastery of Diseart Diarmad in 812 renewing the ties of Bangor with Leinster.
