A Pom is an Englishman, and calling an Irishman a Pom is possibly the most denigrating thing you could possibly say to any Irishman. Off-hand I cannot think of any insult anywhere that rivals it.
We get a lot of Irish fellows working around here and my favourite gambit is to ask them what part of England they hail from, always good for at least twenty minutes of Gaelic profanity...very educational and amusing.
