(The picture enclosed depicts the estatue of Breogan erected in Galicia and his Tower in the background.)
Once in Ireland, they were received by the Tuatha Dé Danann (ancient tribe who occupied Ireland before the Celts) who ambushed the Galicians and managed to kill one of Breogan's sons, Ith.
Years after, Míl Espáine, Breogán's grandson, took vengeance upon the Tuatha Dé Danann and invaded Ireland with the intention of defeating them and settling in the Ireland.
Their invasion features as the main cause of the disapperance of the Irish Tuatha de Danann. According to the Book of the Conquests, the latter were conquered by the Milesians and forced to take refuge under grassy hills, caves, or lands beneath the water... and as their powers dwindled they also dwindled in hight through years of banishment; but still appear as great masters of magic as the Daoine Sidh, and in their old form as Heroic Fairies.
Well, that is what concerns to Faerie, but let us now have a glimpse of Historical facts. This Legendary Irish people, The Tuatha de Danann , who submitted the Fir Bolg and Fomorians, were suddenly conquered by the Spanish Milesians and driven from land in spite of their power and number, by 1800-1300 BC.
According to Wikipedia – The Free Encyclopedia, they are thought to derive from the pre-Christian gods of Ireland.
“When the surviving stories were written, Ireland had been Christian for centuries, and the Tuatha were represented as mortal kings and heroes of the distant past, but there are many clues to their former divine status. Even after they are displaced as the rulers of Ireland, characters such as Lugh, the Morrígan, Aengus and Manannan appear in stories set centuries later, showing all the signs of immortality. They also have many parallels across the Celtic world: Nuada is cognate with the British god Nodens; Lugh is a reflex of the pan-Celtic deity Lugus; Tuireann is related to the Gaulish Taranis; Ogma to Ogmios; the Badb to Catubodua.”
Conclusions: I've outlined two of them ….
1- Historical and archeological facts: denote a relation between the Milesians with the Gaels or Celts from Galicia in Spain. Legends account also tell us about Miletus, an ancient deity of Asia Minor, from where the Sons of Mil invaded Spain eventually arriving in Britain & Ireland. This Invasion was done by ship, leaded by their King Milesius who was Spanish. Thus, legend and history agree about a Human Invasion.
2- Mythological facts: If celtic people considered the Tuatha as Gods, how could it be possible that conquering celtic invaders worshipped the same they conquered and ruled?.
Since that, we may probably consider Milesians as a “Religious Conception” rather than Myth or as Celts themselves.
Legends and Sagas kept by druids gave birth to a Pagan belief never recorded: “The Tuatha” seemed to be early inhabitants that because of their wisdom and prowess of race were heavenly descendant. They were regarded as the Perfect Civilization, and curiously they had their own kings, poets and druids.
Christian scribes always comment that though they “enumerated them” they “ never worshipped” them. And on these subject, the Christian church has much to do. The spread of Christ ‘s word. God’s Promise of Eternity and St. Patrick's labour changed pagan customs for the right Christian ones. People abandoned their Gods and casted them into “Faerie” Realms at the coming of Christianity.
Therefore, the Mythological Paralell tell us about Humans against a Superior Race, humanlike but inmortal. Who at last dwindled into the Daoine Sidh. Christian Culture against Pagan Culture.
I can't assure that Faerie People are descendants of the Tuatha. Perhaps the “Wee Folk” and “Fairies” drifted and long before them and Tuan created “Faerie”, their own realm.
I can only state that recordings on Faerie are more likely to appear on Christian Age recounts than in pre-Christian ones.
I do believe in Faeries as Nature sprits. Due to their motifs and types they range from servicing angel-like beings, to real fallen angels. Modern era and the advance of cities, is also diminishing their realms and faerie lore and beliefs.
Let us keep the legends alive in order to render space for their being.