I believe it does go further back and the problem is inbreeding - like modern Afghans and Pakistanis have.
There was an Irish King/Chief back in the early Middle Ages, I can’t remember what his name was - but I think close to half of the Irish people can claim descendancy from him.
That sort of thing does cause massive issues with genes and mental issues.
You thinking of King Charlemagne? I think fully a third of people of Western European descent are the progeny of Charlemagne. Yours truly included. :-)