Nessa was the daughter of Eochaidh Salbuidhe. She was a female warrior until her wedding to Fachtna. Nessa had a son, Conchobar, and when Fachtna died, his half-brother Fergus Mac Roth was elected to the throne. Fergus was in love with Nessa but Nessa would only agree to become his wife if he would let her son Conchobar rule as king first, for at least a year. Nessa then instructed Conchobar how to be an exceptional monarch and when the time came for Fergus to reclaim the throne, Conchobar was able to refuse his claim with the support of the people. Fergus offered his services and joined with Medb. Later, Nessa's granddaughter was also to become a famous female warrior.
Source: yesss.perso.neuf.fr/Celts/Women/women
Image: Cycle of Irish mythology. She is the enemy of Conchobar mac Nessa via Pinterest