Fiachnae (d. 789), son of Áed Rón and overking of Ulaid, a member of the Cenél Demmáin lineage of Dál Fiatach, was prominent in restoring his dynasty's fortunes and in countering the designs of the powerful Uí Néill kings. His father, Áed Rón (qv) (d. 735) died leaving at least three sons: Bressal, Fergal, and Fiachnae. For the next fourteen years, provincial supremacy rested with Cathussach son of Ailill (d. 749) of the rival Dál nAraide, but reverted to Dál Fiatach in the person of Bressal son of Áed Rón, before his untimely death in 750. By this time Fiachnae had reached mature manhood. His wife's name is not recorded, but he had at least three sons: Eochaid, Cairell (qv) (d. 819), and Loingsech. Succeeding his brother directly, Fiachnae strengthened Dál Fiatach, the ancestral realm of which lay in east Co. Down. Retaining lordship of northern Louth, he extended the territory under the direct control of his dynasty north-westwards to the shores of Lough Neagh. This was achieved at the expense of Dál nAraide and the Cruthin kings of Uí Echdach Cobo. Fiachnae defeated the latter dynasty at Áth Dumai (761). As Dál Fiatach continued to exploit divisions among Dál nAraide, his son Eochaid put their rivals to flight at the battle of Drong (776).
By this time, relationships with the Uí Néill had become increasingly strained. There had been tension ever since 759, when Fiachnae intervened in the dispute between the priest Airechtach (qv) and his rival Fer-dá-Chrích concerning the abbacy of Armagh, slaying a prominent Uí Néill dynast in the resultant battle of Emain Macha. Past grievances on the Uí Néill side, and the consolidation of a Dál Fiatach ascendancy at the expense of other north-eastern dynasties, doubtless provided the context for the rígdál (royal meeting) between Fiachnae and the king of Tara, Donnchad Midi (qv), at Inis na Ríg off the coast of Meath in 784. The choice of a venue in Brega (east Co. Meath) suggests that Fiachnae had been summoned by Donnchad. He may well have been uneasy about the arrangement, as it appears that he remained in his boat throughout the proceedings, refusing to come ashore and treat with Donnchad in person. The outcome of the meeting is uncertain; the records suggest, however, that by the late 780s Fiachnae had to contend with persistent intervention by Donnchad in his sphere of influence in east Ulster.
Throughout his reign, Fiachnae was closely involved in church affairs and patronised Bangor, the foundation of St Comgall (qv). His nephew Diarmait (qv) (d. 825), son of Fergal and grandson of Áed Rón, began his ecclesiastical career at Bangor before settling in Leinster. It seems likely that Fiachnae was responsible for founding the monastery of Dún dá Lethglass (Downpatrick). His son Loingsech died as abbot there in 800. Fiachnae himself died in 789 and the kingship of Dál Fiatach was seized by a distant cousin, Tommaltach son of Cathal. This ‘intruder’ was slain within a year by Eochaid son of Fiachnae, who reigned till 810 and was succeeded by his brother Cairell (d. 819). Most of the later kings of Dál Fiatach descended from Eochaid son of Fiachnae.