In ancient Ireland if a traveler was to happen upon a woman with red hair he must turn around and start his journey all over again. The superstition comes from the legend of the goddess Macha who was said to have cursed the men of Ireland for nine generations with horrible pangs (like labor.)
The curse was not unwarranted of course. While in human form and pregnant the king of the land forced her to race against his fasted horses (for her husband had boasted of her speed) lest he be killed. She begged for sympathy and received none from the warriors in the crowd who were eager to see the sport. She won the race but the stress caused her to give birth to her children in the field. This is still known as Emain Macha (Macha's twins.) For causing her this pain she cursed the men of Ireland except the hero Cuchulainn and her own children. When war came to the land only Cuchulainn could fight.
Macha as it turns out, had red hair.