Speaking as a Briton, albeit one with no strong feelings on either side of the Irish question, I don't have a particular problem with Irish nationalism. I might have a problem if User:Fenian Swine's user page said "Up the 'Ra!" or something like that but it doesn't.
"Fenian" is an English corruption of the Irish word for soldier. However this word has already been incorporated into the largest Irish Republic political party, Fianna Fail ("Soldiers of Destiny" in translation) and the term has become somewhat antiquated as the original Fenians were operating in the mid-19th century.
I think when considering a politically POV username, regard should also be had to the user's edits. If the user is in constant trouble for highly POV editing then it's more appropriate to ask for a name change, but that does not appear to be the case here.
David Boothroyd [[en:User:Dbiv]]