Ith was the son of Breogan of Spain and the brother of Bile. Breogan had built a tower overlooking the sea, and one day Ith saw Ireland fron the top of it. Curious about this, Ith gathered a group of followers and set off for Ireland. When Ith arrived, he met the three kings of the Tuatha de Danann, Mac Cuill, Mac Cecht, and Mac Griene. They welcomed Ith at first, but then they became suspicous about why he had come to Ireland and murdered him.
Ith's followers managed to escape with their master's body and return to Spain. Mil Espaine (Ith's nephew) had died while Ith was jouneying to Ireland, and when Mil's sons, Erimon, Amergin, and Eber Finn discovered that their great uncle had been murdered, they figured there was no poin in continuing to live in Spain. And thus, the murder of Ith caused the sons of Mil (the Gaels) to invade Ireland.
Ith was the father of a son named Lugaid mac Itha, who was the husband of Fial, daughter of Mil. Lugaid probably went with Mil's sons to Ireland, as he must have been eager to avenge his father's death.