The Lebor Gabála, which was written in the 11th century AD by Christian monks, purports to be a history of Ireland and the Irish (the Gaels). It tells us that all mankind is descended from Adam through the sons of Noah, and that a Scythian king named Fénius Farsaid (descendant of Noah's son Japheth) is the forebear of the Gaels. Fénius, a prince of Scythia, is described as one of 72 chieftains who built the Tower of Babel. His son Nel weds Scota, daughter of an Egyptian pharaoh, and they have a son named Goídel Glas. Goídel crafts the Goidelic (Gaelic) language from the original 72 languages that arose after the confusion of tongues. Goídel's offspring, the Goidels (Gaels), leave Egypt at the same time as the Israelites (the Exodus) and settle in Scythia. After some time they leave Scythia and spend 440 years wandering the Earth, undergoing a series of trials and tribulations akin to those of the Israelites, who spent 40 years wandering in the wilderness. Eventually, they reach Hispania by sea and conquer it. There, Goídel's descendant Breogán founds a city called Brigantia, and builds a tower from the top of which his son Íth glimpses Ireland. Brigantia refers to Corunna in modern-day Galicia, in Spain, (which was then known as Brigantium) and Breogán's tower is likely based on the Tower of Hercules, which was built at Corunna by the Romans.
Íth sails to the island with a group of men. He is welcomed by its three kings: Mac Cuill, Mac Cecht and Mac Gréine. These three are members of the Tuatha Dé Danann, who ruled Ireland at the time. Evidence suggests that Tuath Dé were the main pagan gods of Ireland. Íth is then killed by unnamed attackers and his men return to Iberia. The eight sons of Íth's brother Míl Espáine (whose given name was Galam), lead an invasion force to avenge his death and take Ireland. After they land, they fight against the Tuath Dé and make for Tara, the royal capital. On the way, they are met on three mountains by Banba, Fódla and Ériu – the wives of Ireland's three kings. They are believed to have been a trio of land goddesses. Each woman says that the Gaels will have good fortune if they name the land after her. One of the Gaels, Amergin, promises that it shall be so. At Tara, they meet the three kings, who defend their claim to the joint kingship of the land. They ask that there be a three-day truce, during which the Gaels must stay a distance of nine waves from land. The Gaels agree, but once their ships are nine waves from Ireland, the Tuath Dé conjure up a great wind that prevents them sailing back to land. However, Amergin calms the wind by reciting a verse. The surviving ships return to land and the two groups agree to divide Ireland between them. The Gaels take the world above, while the Tuath Dé take the world below (i.e. the Otherworld) and enter the sídhe, the ancient burial mounds that dot the Irish landscape. Amergin divides the kingship between Éremon, who rules the northern half of Ireland, and Éber Finn, who rules the southern half. This division of the land was probably invented by the writers to explain and justify the 7th/8th-century division between the royal capitals of Tara and Cashel. The Lebor Gabála then traces Ireland's dynasties back to Milesian Gaels such as Éremon and Éber. Modern scholars, however, believe that these were fictional characters and that the writers were attempting to give the medieval dynasties more legitimacy.
The Historia Brittonum—which was written before the Lebor Gabála—gives another account of the Milesians. It says that "three sons of a Spanish soldier" sailed from Iberia to Ireland with thirty ships, each carrying thirty wives. They see a glass tower in the middle of the sea with men on top of it, but the men do not answer their calls. The Milesians set out to take the tower, but when they reach it, all but one of their ships are sunk by a great wave. Only one ship makes it to land, and its passengers are the ancestors of all the Irish. In the Lebor Gabála, it is the earlier Nemedians who are drowned while trying to take a tower.