ok, where did you get that?
You can read all about the Galician invasion of the place we now call Ireland at a number of places on the net. The best site has all the most ancient works translated into modern Spanish!
You can dig through the morass of stuff to find it ~ so much more now than nearly a decade ago when I first found it. Best way to start your search is to look for "Ir Scota Dead Coast". Should get you a few websites to look at.
Note the reference to "pseudo history". These are documents maintained by the decendants of formerly Gaelic speaking people in Galicia in NW Spain (their language died out in the 16th century and was replaced with 3 versions of Spanish).
Back in distant times when the English ran the world for the Irish it was believed the documents were just so much BS because they're written in Greek ~ later research has determined that the Mediterranean/Black Sea Celts were predominantly a sea-going people and the regularly used Greek scribes ~ as did their contemporaries. Even though their own language wasn't put to writing until Roman times, their second language was, in fact, Greek, so these are the records, and myths, and so forth.
They derive from the time BEFORE the people who gave Scota its name moved there. (Scota is, of course, modern Ireland. It received it's name when the nation/culture that founded Scota relocated to Alba, now known as Scotland in the 7th - 9th centuries).
The name "mil", as in "Milesian" means "Man".
Have fun. (Whoops, important news ~ depending on which side you take ~ the Republican or Royalist, in the Spanish Civil War you will find discussions of these documents that step off the planet one way or the other.)