Ireland had five waves of invaders, according to traditional history:

Cessair, the granddaughter of Noah, brought a ship with 50 girls and 3 men to Ireland. All of them died, mostly in the Flood, except for Cessair’s husband, Fintan.

The Partholons fled to Ireland from Greece after a failed attempt to take over the throne. They died in a huge plague.

The Nemedians were next, and most of them died in battle with the Fomorians or from the plague. About thirty survived, and most of them fled to other countries.

The Fir Bolg is what we call three groups, all descended from the Nemedians, who returned to Ireland. They were small, hard-working people, and their name translates either as “the bag men” or “the men of lightning.”

The Tuatha De Danann arrived while the Fir Bolg were still ruling Ireland, but the Tuatha De Danann (“children or tribe of the goddess Danu”) defeated the Fir Bolg in battle. The Tuatha De Danann, described as “gods and not-gods” had magical powers and ruled Ireland for many years. They were the fifth invaders.

The last to land in Ireland were the Sons of Mil (also called the Milesians), probably from Spain or the Basque country. These are the Celts. They had one battle with the Tuatha De Danann, and the Tuatha De Danann retired to “the hollow hills” (or the faerie world) where they live today. The Milesians are regarded as modern Ireland’s ancestors.

One group  is not included in this traditional history:  The Formorians.  They were in Ireland at the time of the Partholons, but we know very little about them.