Conchobar mac Nessa (son of Ness) is the king of Ulster in the Ulster Cycle of Irish mythology. He rules from Emain Macha (Navan Fort, near Armagh). He is usually said to be the son of the High King Fachtna Fáthach, although in some stories his father is the druid Cathbad, and he is usually known by his matronymic, mac Nessa: his mother is Ness, daughter of Eochaid Sálbuide, King of Ulster.
There are several versions of how Conchobar was conceived. In the earliest, Ness, daughter of Eochaid Sálbuide, the then king of Ulster, asks the druid Cathbad what it is an auspicious time for. Cathbad replies, "for begetting a king on a queen". There are no other men around, so Ness takes Cathbad to bed and she conceives a son. In a later version, Ness is brought up by twelve foster-fathers, and while all twelve are at a feast, Cathbad, leading a fian or landless war-band, attacks the house and kills them all. Eochaid is unable to avenge them as the culprit cannot be identified, so Ness forms her own fian to hunt Cathbad down. But while she is bathing alone in a pool, Cathbad appears, stands between her and her weapons, and bares his sword. He spares her life on the condition that she becomes his wife. They settle near a river called Conchobar, and Ness soon conceives a son, but in this version, the father is the High King Fachtna Fáthach, who is Ness's lover. As she and Cathbad set out to visit Fachtna, Ness goes into labour. Cathbad tells her if she can manage not to give birth until the following day, her son will be a great king and have everlasting fame, for he will be born on the same day as Jesus Christ. Ness sits on a flagstone by the river Conchobar, and the following morning gives birth. The baby falls into the river, but Cathbad lifts him out, names him Conchobar after the river, and brings him up as his own son.
By the time Conchobar is seven, Fergus mac Róich is king of Ulster and falls in love with Ness.