The Irish race of today is popularly known as the Milesian Race, because the genuine Irish (Celtic) people were supposed to be descended from Milesius of Spain, whose sons, say the legendary accounts, invaded and possessed themselves of Ireland a thousand years before Christ.
The races that occupied the land when the so-called Milesians came, chiefly the Firbolg and the Tuatha De Danann, were certainly not exterminated by the conquering Milesians. Those two peoples formed the basis of the future population, which was dominated and guided, and had its characteristics moulded, by the far less numerous but more powerful Milesian aristocracy and soldiery. All three of these races, however, were different tribes of the great Celtic family, who, long ages before, had separated from the main stem, and in course of later centuries blended again into one tribe of Gaels - three derivatives of one stream, which, after winding their several ways across Europe from the East, in Ireland turbulently met, and after eddying, and surging tumultuously, finally blended in amity, and flowed onward in one great Gaelic stream.
The possession of the country was wrested from the Firbolgs, and they were forced into partial serfdom by the Tuatha De Danann (people of the goddess Dana), who arrived later. Totally unlike the uncultured Firbolgs, the Tuatha De Dannann were a capable and cultured, highly civilised people, so skilled in the crafts, if not the arts, that the Firbolgs named them necromancers, and in course of time both the Firbolgs and the later coming Milesians created a mythology around these.
In a famed battle at Southern Moytura (on the Mayo-Galway border) it was that the Tuatha De Danann met and overthrew the Firbolgs. The Firbolgs noted King, Eochaid was slain in this great battle, but the De Danan King, Nuada, had his hand cut off by a great warrior of the Firbolgs named Sreng. The battle raged for four days. So bravely had the Firbolgs fought, and so sorely exhausted the De Dannann, that the latter, to end the battle, gladly left to the Firbolgs, that quarter of the Island wherein they fought, the province now called Connaught. And the bloody contest was over.
The famous life and death struggle of two races is commemorated by a multitude of cairns and pillars which strew the great battle plain in Sligo - a plain which bears the name (in Irish) of "The plain of the Towers of the Fomorians". The Danann were now the undisputed masters of the land. So goes the honoured legend.