Fiachrae was an Irish prince, the son of the high king Eochaid Mugmedón (d.362)[1] by his wife Mongfind, sister of Crimthann mac Fidaig (d.367).
According to this saga, his half-brother the high king Niall Noigiallach (d.405) made Fiachrae's full brother Brión his champion and Brion seized the kingship of Connacht.
This led to war between Brion and Fiachrae and they fought a battle at Damchluain (near Tuam, County Galway).
However Fiachrae's son Nath Í rallied forces and defeated and slew Brion at a second battle at Damchluain.
Fiachrae was now released by Niall and given Brion's position as champion and levier of his hostages and rents.