The racial slur of the day is Mackerel Snapper.

It is used to refer to Irish.

Deep lore: It can be applied to any Catholic, Irish or otherwise. In the past, Catholics were forbidden from eating meat on Fridays. They got around this sort of the same way some vegetarians do, by eating fish (although technically this is not vegetarian, but "pescetarian"). Still observed in heavily-Catholic areas (they even have school/church fish-fry Fridays).