Well as to the dark hair in the Irish
there was a tradition that it came from wrecked Spanish sailors who married into the population. So you'd have clans with dark hair i.e. the Black Mulvihills. I'm pretty sure that Scotland has the highest proportion of redheads in the population- 13% around I think. On the other hand travel has meant there has been enough intermarriage that distinctions are probably beoming fewer.

Checked wikipedia (font of all knowledge) and apparantly Scotland and Ireland have the highest rates in the world, and from own knowledge of history red hair was common amongst the Celts

Edit: oh and as to the blond/red hair point, I'm pretty sure the two aren't linked. Red hair has its own gene, and is not dependent on whether you are blond or not- two blond haired people won't have a red headed child (unless they each have a recessive redhaired gene). So higher rates of blond hair won't lead to more red heads in theory at least
( Amberl was stripey and dominated Europe, Wed 3 Mar 2010, 11:10, Reply)