I have a geographical/cultural question about Ireland.?

Most Irish people have dark hair. There isn't one specific group or region. You might be thinking of the term "black Irish", but that's a term coined by Irish Americans to describe Irish people that don't fit the red hair stereotype. Other people, even in Ireland, claim that there are dark people in Ireland descended from sailors of the Spanish Armada in the 1500s. It's possible that some Irish people are, but it's so long ago that it wouldn't make much difference to how people look today. (And certainly not in medieval times, which was before the Armada.)