Cathbad also features in both accounts of Conchobar's birth, one of which details him as the King's father. In the first, Nessa, daughter of Eochaid Sálbuide, the then king of Ulster, asks the druid what it is an auspicious time for (as he had the ability to foretell the future). Cathbad replies, "for begetting a king on a queen". There were no other men around, so Nessa takes Cathbad to bed and conceives a son.
In the second version, Cathbad, who is a leader of a band of fianna (landless warriors) as well as a druid, attacks Nessa's foster-fathers' house, killing all twelve of them. Because the culprit cannot be identified, Eochaid is powerless to do anything about it, so Nessa forms her own band of 27 fianna to track him down. However, one day, when she goes off on her own to bathe, Cathbad comes upon her alone and unarmed and demands her as his wife. She has no choice but to agree. However, in this version, Nessa's child is the son of Nessa's lover High KingFachtna Fáthach, as oppressed to Cathbad's. Regardless though, Nessa goes into labour and 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. Nessa 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.
The druid was also was present at the birth of Deirdre and prophesied her tragic fate, but Conchobar ignored him. On another occasion, the young Cúchulainn overheard Cathbad prophesies that anyone who took arms on that day would have everlasting fame but a short life; he immediately ran to Conchobar and asked to be armed.