Used by people not living in Ireland or Northen Ireland to offend the inhabitants. If the person being abused is a Catholic then he will be offended that someone not of his blood is calling him thus and is most likely to twat the offender. If aimed at a protestant (usually from Northern Ireland) then he will be vexed by the fact he is being compared to a Fenian and is most likely to twat the offender.
See you fenian cunt, i'll cut you first!
Nice on nancy, cop a load of this!