When the Vikings conquered the British Isles and established settlements in the late 700's and early 800's, they brought with them the red hair gene. Today, 10 to 30 % of Irish people have the red hair gene; 10 to 25% of Scotland; and 10 to 15 % of Welsh people. If you examine where the Vikings settled in Ireland (Limerick, Cork, Wexford, Dublin) you see the highest concentration of redheads. While the Danes, associated with blond hair, conquered parts of the British Isles, the red hair is traced more so to Norway. Remember Eric the Red, the first Norse settler of Greenland? He was named for his red hair and beard. Redheads have the stereotype of being hot-tempered. Maybe this trait originates in their Viking roots.
According to one website, the dividing line for red hair seems to be the 45th parallel which runs through Central France, Northern Italy and Croatia. The 45th parallel sits exactly half way between the equator and the North Pole. South of the 45th parallel, the UV rays become so strong that it is no longer beneficial to have red hair and fair skin. Ireland, which sits on the 53rd parallel is definitely north of the dividing line.
Ireland offers several redhead conventions to cater to the ginger population including festivals in Cork http://www.redheadconvention.com/, To learn more about redhead stereotypes, visit http://gingerparrot.co.uk/facts-and-myths-about-red-hair/.