"The Thirteenth Son of the King of Erin" is an Irish fairy tale collected by Jeremiah Curtin in Myths and Folk-lore of Ireland.
He asked his father for an outfit for the road, and the king gave him it and a black horse that could run faster than the wind.
One day, he put on some poor clothing and was hired by a king to herd his cows.
They promised their swords of light and horses if he would spare them, but he killed them, and their housekeepers, glad to be freed, showed him all their treasures.
He cut the sea serpent in half, but the halves joined again, and it threatened that no one would save her the third day.
The third day, he wore the many-colored clothing of the third giant, with its blue glass boots, and rode its red horse.
When he was dressed the housekeeper told him that no one could fight the sea serpent that day; the only way to defeat him was to throw the brown apple she gave him down its open mouth.
They sent twenty men for him, but he overpowered them; when they sent another twenty, he overpowered them as well; finally, the seer told the king to go himself, and when the king asked and told him not to mind his work, Seán Ruadh came.