I think genetics is obviously the reason for red-headed prevalence in Ireland/Scotland, along with long-term environmental influence.  But I guess I hadn't ever really wondered what might account for the difference between blonds and redheads.  It seems natural that light-haired and light-skinned people of northern Europe simply did not need the darker skin pigments of people living in warmer climates.  Why red hair sometimes, and why blond at others?  Perhaps the same reason why there are so many different-colored dogs in the world.  I remember watching a show which explained that as generations of dogs became more tame, and they mated with other tame dogs, the characteristics of these animals changed.  This is why you can get a lovable basset hound within the dog family as well as a fierce pit bull or rottweiler.Perhaps this was similar with humans....not about them being "tame" but about people reproducing who shared common characteristics, diet, etc.

Red hair could indicate a genetic connection with Neanderthals who also had the genes for red hair.http://www.sciencedaily.com/releases/2007/10/071025143311.htm