One of the passages in the Lebor Gabála Erénn that is sometimes argued about is what was meant by the claims the Milesians came from Spain. Some considered it possible, others weren't sure. The question appears to have been settled, alongside any claims that there is such a thing as a unique strain of genetics that could be called Celtic.
Bryan Sykes, the man who worked out the Daughters of Eve research that demonstrates 95% of humanity descends from seven women based on DNA markers alongside many other accomplishments, spent the last several years researching the genetic history of Great Britain and Ireland. What he found supports the LGE's claims of the origins of the Milesians. Genetically, the large majority of residents of those two islands can be traced back to the northern coast of Spain. About 6,000 years ago, the civilization on that section of the European mainland were shipbuilders of sufficient talent that their crafts made it to those islands. They weren't the first humans there; some small population already existed there but was mostly subsumed by the newcomers. Successive waves since then that left their marks were the Saxons, Normans and others. Also, the markers for the first wave are so consistent across the two islands that it is foolhardy to claim a genetic difference between a Londoner and a Dubliner without one of them being non-white. Or perhaps not. Interestingly, the probable source of the "black Irish" also seems to have been located. Around the same time those Iberians were making the journey (the Oisin genetic marker in Sykes' nomenclature), some people from northern Africa were there along with them and settled primarily in the coastal areas (labeled the Eshu marker).
I look forward to getting my hands on his book, which is being published as Blood of the Isles in the UK and Saxons, Vikings and Celts in the USA. I dearly wish my paternal line had the Irish and that I had the spare cash to take advantage of the genetic testing available through his institute's site. But those of you who do may find it worth a look.