Cú Chulainn’s mother is Deichtine, who becomes pregnant by the God Lugh who plants a child in her womb, instructing her to call him Sétanta. However, because she is betrothed to Sualtam mac Róich, and her own father King Conchobar is suspected of being the parent, she aborts the child and goes to her husband's bed. She then conceives a son whom she does name Sétanta.
Cúlann the smith invites Conchobar to a feast at his house. On his way, Conchobar sees Sétanta hurling with a group of boys. Being impressed he asks Sétanta to join him at the feast after his game has finished. Conchobar later forgets and Cúlann has his ferocious hound protecting his house. Sétanta arrives and is attacked by the enormous hound and kills it in self-defense. Cúlann is devastated, so Sétanta promises to raise a replacement and until then, he will guard Cúlann's house. Cathbad the Druid declares that Sétanta will henceforth be called Cú Chulainn, meaning "Cúlann's Hound."
After overhearing Cathbad declare that anyone taking up arms one partiCúlar day would that day would achieve everlasting fame, Cú Chulainn.at just seven years old asks to be armed. Despite his age, none of the weapons withstand his strength until King Conchobar gives him his. Cú Chulainn proves fierce and merciless in battle, sometimes transforming with warp spasm and a rage so terrible that it had to be calmed when he returned home.
Cú Chulainn only has eyes for one woman Emer and after she refuses the hand of Lugaid, a king of Munster and he finally kills her over protective father, they are joined in marriage. Although Cú Chulainn had many lovers, Emer's only jealousy is when he falls in love with Fand, wife of Manannán mac Lir. After Manannán has left her she was attacked by three Fomorians. Cú Chulainn agrees to help as long as she marries him. Cú Chulainn is mortal, however, and Fand is Fae. Emer tries to kill Fand, but after seeing the strength of their mutual love she decides to give him up to her. Fand, touched by Emer, decides to return to Manannán, who uses a potion to wipe the affair from their memories.