some of the studies have theorized due to the famine but I think it probably is further back than that.
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.