Particularly with Irish people it's more common. Theirs a few genetic pathways to the light skin common in more northern European areas. One of them showed up early in human migrations, and is pretty directly connected to production of the darker skin pigments. Adaptation done, no need for further tweaking to get vitamin D synthesis advantage.
These people tend to have irregular patches of darker melanin regions, hair containing only the lighter variety of melanin, and skin that burns easily because the mutation provides less gradient of melanin production, and more of a "yes or no".
As a result, the path for that gene to be deactivated is also shorter and it's more likely for people to have kids who don't get it nor have many of the genes for the other ways for skin to be lighter.

There aren't a huge number of genes that act like switches like that, so it's very startling for people.