For those hosting the party, it probably has some religious significance. Are you asking because you want to party? Do we not have enough holidays of our own, do we have to celebrate extra goyish holidays? Can’t you hold out for another week till Purim?
If it is a business situation IMHO tell them that you can’t really make it but you will try to stop in to say hello. If it is friends let them know that you don’t celebrate St. Patricks day and unless you are “lunching” in a glatt kosher restaurant you really can’t attend.