You thinking of King Charlemagne? I think fully a third of people of Western European descent are the progeny of Charlemagne. Yours truly included. :-)
No, it’s some Irish Chief or King of Ireland (circa 300AD - 600AD IIRC).
They made a big deal about Obama and Biden being “semi-related” to each other back in 2008 because both descended from this guy I’m referring to. Apparently he got around more than Solomon and King Henry VIII combined.