Conchobar mac Nessa was the king of Ulster in the Ulster Cycle of Irish mythology. He ruled from Emain Macha. He is said to be the son of Cathbad the druid, although unusually his descent references his mother, Ness, daughter of Eochaid Sálbuide, king of Ulster.
His mother, 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. 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 a great unknown saviour of mankind. 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. She agrees to become his wife, on one condition: that Fergus allows Conchobar to be king for a year, so his children will be called the sons of a king. The nobles of Ulster advise Fergus that this will not affect his standing with them, as the boy will be king in name only, so he agrees. But Nessa, using her son as a public front, rules so well that by the end of the year it's decided he should be king permanently. To keep a hold of her son, she marries him and bares him a son - Cormac Cond Longas.
Fergus makes an alliance with the new High King, Eochu Feidlech, and they make war on Ulster. After a series of bloody battles, Conchobar makes overtures for peace. Fergus is offered land, the Champion's Portion at Emain Macha, and the position of Conchobar's heir. Conchobar demands compensation from Eochu for the killing of his father, Fachtna Fáthach, and is granted land, status and the High King's daughter in marriage. His second wife Medb however walked out on him, giving him a lifelong grudge and an unfortunate disposition.
Afterwards without his mother to keep him in line, he made a series of increasingly poor decisions that lead to the Tain Bo Cuailnge.
Conchobar is later injured by a wound inflicted by a Connacht warrior. His doctors are unable to fix the wound and settle with sewing up the area on his head. They warn Conchobar that he must not get too excited or over-exert himself or he will die. Seven years later, he is told of the death of Christ and is so angered, his wound burst and his head exploded.