[Ir. fáthach, possessed of knowledge; wise, sagacious].
A giant, king of Ulster, sometimes described as the husband of Ness and the father of Conchobar mac Nessa. The better-known version has Cathbad the druid impregnate Ness to become the father. After Fachtna's death his half-brother Fergus mac Róich marries Ness and succeeds him as king.