Before coming to Ireland, he and his brother Éber Donn were joint rulers of Spain. His great-uncle Íth made a peaceful expedition to Ireland, which he had seen from the top of a tower built by his father Breogan, but was killed by the three kings of the Tuatha Dé Danann, Mac Cuill, Mac Cecht and Mac Gréine, and in revenge the Milesians invaded in force, with Érimon and Éber Donn in command. They defeated the Tuatha Dé in the Battle of Tailtiu. Éber Donn had been killed, and the High Kingship was divided between Érimón in the south and his younger brother Éber Finn in the north.
Érimon had two wives, Odba, mother of Muimne, Luigne and Laigne, whom he left behind in Spain, and Tea, mother of Íriel Fáid, who accompanied him to Ireland, and died there. Tea gave her name to Tara, where she was buried - the Lebor Gabála Érenn explains its Old Irish tame Temair as "Tea mur", "Tea's Wall".
Some accounts list this second wife as Tea Tephi, daughter of Zedekiah the last king of Judah, who escaped the destruction of Jerusalem by the Babylonians and after stoping in Egypt, sailed to Ireland.
A year after the Battle of Tailtiu, Éber Finn became unhappy with his half, fought a battle his brother at Airgetros, lost and was killed. Érimón became sole ruler of Ireland. He appointed kings of the four provinces. He gave Leinster to Crimthann Sciathbél of the Fir Domnann; Munster to the four sons of Eber Finn, Ér, Orba, Ferón and Fergna; Connacht to Ún and Étan, sons of Uicce; and Ulster to Eber mac Ír. During this time the Cruithne settled in Ireland. He ruled for fourteen, fifteen or seventeen further years, after which he died at Airgetros, and was succeeded by his sons Muimne, Luigne and Laigne, ruling jointly.