Map showing frequency of red hair across Europe. Five color-coded ranges: red (over 10%) concentrated in Ireland and Scotland, orange (5-9%) in parts of UK and northern Europe, brown (2-4%) in Scandinavia and Germany, darker brown (1-2%) in central Europe, and darkest brown (under 1%) in southern Europe. Shows clear north-south gradient with highest concentrations in Celtic regions.
Somebody with red hair had a lot of children there right before the population suddenly grew. Or maybe was one of the survivors when it regrew after a national disaster. The founder effect, in other words.
Other people are talking about selection. The numbers are in, that accounts for maybe 5% of genetic change over the last 10,000 years. The rest is just chaos.