I had no idea about what this video shared.
First delusion that it shattered. The Irish are not Celtic. In fact, they are a whopping 3 times less Celtic than the English! (5% vs 15%)
Second delusion is about the Dark Ages. The period directly after Rome fell. We’ve all been taught that the people of this time regressed into deep conservatism, with the average person living in a hyper-patriotic and bellligerently prjudiced albeit tiny little kingdom, ruled by some jealous royal family that played into people’s ready patriotism. And then, that Europe became more liberal in the enlightenment, which would include more openness to the outsiders, tolerance of foreigners, cultural mixing, etc.
The opposite, once again, is true. The Dark Ages were a period of rapidly shifting borders and political allegiances, and more than anything, it was the period of the greatest amount of migration within Europe! The DNA of Europe was constantly being mixed during the Dark Ages, and peoples’ settled towns were constantly being flooded with wave after wave of peoples speaking other languages, with other little traditions and fixations.
The end of the Dark Ages was the establishment of large, stable nations, where previously shifting multicultural societies were forced to consolidate one national identity: French, German, English, etc.
That consolidation involved standardization of religion and enforcement of land rights - and with that, borders.
We should question whether the success and progress of the post-Dark Ages is thanks to the social stability and therefore social trust that was created by enforcing common identity and low-immigration.