Legend has it that the families and clans of Ireland descended from the three sons of King Milesius of Spain and are known as "Milesians". Prior to this, thousands of years ago, it is said that Ireland was inhabited by two tribes – the Tuatha de Danann and the Fir Bolgs. An uncle of King Milesius named Ith, on an exploratory journey to the emerald island, encountered the chieftains of the Tuatha de Danann and was slain.
King Milesius was outraged at the death of his uncle and sent his sons, nine in number, to Ireland with a great fleet to avenge his death. On landing in Ireland the sons of King Milesius went inland and there met the kings of the Tuatha de Danann, demanding of them either kingship or battle. The kings of the Tuatha de Danann stalled for time, asking for a week alone on the island before making a decision. To this the sons of King Milesius agreed. They then returned to their ships and sailed a short distance off the coast of Ireland. The treacherous Tuatha de Danann then raised a great druidical storm against the Milesian fleet, which drove them far to the west. They circled the island three times until the storm blew itself out, finally landing in the south of the island. Here they divided their fleet and men, Heber, the oldest son still living (most of the sons of Milesius had been killed in the landing or the storm), remained in the south of Ireland. Heremon, his brother, and the rest of the fleet sailed to the north, where they landed their ships. Coming inland the sons of King Milesius again joined their forces and engaged the Tuatha de Danann in battle, completely routing them and slaying all their leaders.
All of the sons of King Milesius were slain in the conquest of Ireland except for Heber and Heremon. Heber Finn, the son of Ir, survived, as did Lugaidh, the son of Ith. From the three sons of King Milesius to have issue, namely Heber, Ir and Heremon, and from Ith, King Milesius' uncle, are said to descend the great clans and families of Ireland, known as "Milesians," in honor of their great ancestor, King Milesius of Spain.
After conquering the island Heber and Heremon divided Ireland between them. To Heremon went the northern half of the island and there his descendants are mainly to be found to this day, including the northern and southern Ui Neill, King of Meath and Ulster, the Ulaid, the Dal Riada (who later founded the kingdom of Scotland) and the Kings of Leinster. From Heber are said to descend the tribes and kings of the south of Ireland. Heremon gave a part of his kingdom to Heber Finn, the son of his slain brother, Ir, and from him are said to descend the Knights of the Red Branch in Ulster, Clanna Rory. From Ith, King Milesius' uncle, are said to descend some of the tribes living in the province of Connacht.