Cu Chulainn gained his better-known name as a child after he killed Culann's fierce guard-dog in self-defence, and offered to take its place until a replacement could be reared.

At the age of seventeen he defended Ulster single-handedly against the armies of queen Medb of Connacht in the epic Táin Bó Cúailnge ("Cattle Raid of Cooley").

It was prophesied that his great deeds would give him everlasting fame, but that his life would be a short one. This is the reason why he is compared to the Greek hero Achilles.