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.
Looks like radius from Belfast and Dublin, the big cities, which had more immigration from the uk. However, that doesn’t explain north versus south which should skew differently if that was the case.
The southern part is Cork, the second biggest city in the republic. Dublin and Cork are where Norse and then British settlement was most concentrated over time as well