My friend is visiting from Ireland and wants to know what she would use as a zipcode at the gas pump in USA when in Ireland they don't have a zip code ???
You are misunderstanding the system.
You think that you give a card and a zip-code and the machine says the bank, "Is Jon Kline good for $13.50?" and the bank says back, "Yes, if he knows that his zip code is 12345."
No, the way it works is the machine says to the bank, "Is Jon Kline who resides in the 12345 zip code good for $13.50?" and the bank says yes or no.
Your bank, being in Ireland, where they do not have zip-codes, is likely to disregard the zip-code and approve the sale, or not, on other criteria.
tl;dr: Enter 00000.