Spit in his Guinness
Spitting in his beer is bad enough – but everyone knows that an Irishman’s Guinness is sacred, and not to be tampered with. The ultimate insult to an Irishman.
Insult his mother
Ok, it is certainly true that insulting one’s mother in any culture is pretty bad. And so in this instance, the Irish are no different to the Spanish or the Italians. (In fact, it may even be more serious in these countries.) Nevertheless, given the close relationship that tends to exist between the Irishman and his mother, you really can’t go wrong with this time honored old insult.
Insult his mother's cooking
Related to above, except in many ways even worse, because it can cut especially close to the bone. For example, if you say, "You're mother is a prostitute," many Irishmen may be able to brush this off because they know its simply not true. (Unless of course their mother really is indeed a prostitute.) But, on the other hand, if you tell an Irishman, "You're mother is a lousy cook," while perhaps not at first looking as serious as the previous insult, can, in fact, be far more offensive. More often than not, there may be some truth behind this last insult that is lacking in the previous one. Because, let's face it, Irish cuisine, compared to other countries, ranks fairly poorly. Putting Irish food in the same sentence as the cuisine of France, Italy or Spain makes as much sense as comparing the Irish soccer team to the soccer teams of those three European countries.
Tell him U2 are overrated
A minority of Irish people might actually agree with you – but the rest will smack you down hard and go on at length about how seeing U2 play "Where the Streets Have no Name" live is a "spiritual experience."
Ask him if Ireland is still part of Britain
No, it’s not. (Yawn...) And nor has most of Ireland been part of Britain since 1921. And no, Ireland (the republic, that is) is not ruled by the Queen of England. This can be a fairly touchy subject. Just as the best way to annoy a Canadian is to confuse him for an American, and the same for a New Zealander with an Australian, a terrific way of getting under an Irishman’s skin is to confuse him for an…yes, you’ve guessed it.
Demand his pot of gold
This will inevitably invite a sigh of tedium. For some reason, Irish-Americans – rather than Irish people – have a fixation with this mythical creature. Irish people are largely indifferent to it, especially now that it's been extinct for so long, and will remain so until a team of scientists, inspired by Jurassic Park, work a little magic with some Leprechaun DNA. But Leprechaun references annoy them none-the-less. (This one works better if you shake the Irish person really hard as you demand his pot of gold.)
Insist that he dance a jig
Surprisingly enough, not all Irishmen can do Irish dancing. In fact, most Irishmen never do any form of dancing, Irish or otherwise, especially if they are sober. It will certainly annoy any Irishman if you repeatedly ask him to "do the Michael Flatley." For added annoyance, throw quarters at as him as you ask him to do this.
Ask him if people still marry their cousins in Ireland
As with Insult No.3, this one can be an effective insult precisely because it can cut a little too close to the bone. (Especially if the Irish person in question is from County Kerry.)
Tell him that the ancient Irish sport of hurling is basically an amateur version of hockey
In a review of the movie "The Wind that Shakes the Barley," the British paper, the Sunday Telegraph, described a game of hurling as "amateur hockey." Never mind that hurling - a game which looks like a cross between hockey and lacrosse - in fact predates hockey (and lacrosse for that matter) by many, many years. This insult, or breathtaking ignorance, will be enough for any Irishman to come after you and want to beat the bejasus out of you with his hurley. (Which although looks like something like a hockey stick, IS NOT a hockey stick.)