In Irish mythology Partholon was the leader of the second group of people to settle in Ireland, the first to arrive after the biblical Flood. They arrived from the west 312 years after Cessair and her followers.

Partholon was the son of Sera, who was the king of Greece. Partholon fled from Greece, after murdering his own father and mother. He lost his left eye in the attack on his parents. Accompanied with his wife Dealgnaid and a group of followers, they reached Ireland after wandering for seven years.

On the fourteenth day of May (a Tuesday), Partholon is said to have landed at Kenmare in West Kerry. He brought with him his wife and their three sons Laiglinne, Sláinge and Rudraige and and their wives, and a host of a thousand along with them. When they arrived there was only one plain in Ireland, but Partholon cleared four more. There were seven lake-bursts in his time.

They settled for a while on an island in the middle of Lough Erne, which they called Saimher after a lap-dog that Partholon had killed out of jealousy of his wife.

It seems that Partholon's wife had had an affair with her attendant Todhga and made no apology for it. Saying: O Partholon, is it possible for a woman to be near honey, or a child next to new milk, or a cat smell fresh meat, or a workman see sharp tools, or a man and woman be close in private, without meddling the one with the other? and in anger Partholon killed her little dog Saimher. This is recounted as the first jealousy in Ireland.

They encountered the Fomorians, whom they fought in battle. The Partholonians managed to defeat and drive the Fomorians from Ireland.

However, Partholon died, after 30 years in Ireland. The rest of the Partholonians died 120 years later from pestilence. This plague took hold and laid waste to five thousand men and four thousand women in one week in the month of May.