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.
The Fenians were members of the Irish Republican Brotherhood (IRB) in Ireland in the 1860s. They are essentially the pioneers of modern terrorism, and used violent methods in order to gain what they wanted - in this case they wanted Ireland to be made a Republic, separate from Britain and with their own Parliament.
A word that describes Irish Catholics, or Irish American Catholics, who are mostly Irish Republican, native to Ireland, and are Catholic. Fenian can be used as a slur to insult an Irish Catholic for their struggles in the Troubles in Northern Ireland, being discriminated against by the Loyalist Protestants who hated Irish Catholics.