Why does it irritate you though? There are all sorts of schools of thought on all sorts of things out there from sex and fidelity to cars, to education, to religion and so on.I do not have a problem with people who choose to have a meat/dairy etc free diet. It is the cult 'Veganism' that irritates me.
You have to ask yourself why someone being a vegan irritates you. If you have a religious view, does someone with a different view irritate you? Why so?