I remember a time before Ireland set up postcodes (zip codes) for the whole country. If postcode was mandatory field in an e-commerce address form, you couldn't mail stuff from the UK unless it was in Dublin. Dublin had postcodes.

I managed to find one site that would accept 'null' so the form would submit.

Ireland's postcode system now is a thing called Eircode, which each eircode maps to your address exactly

I used to just do "n/a", "na", "none", or "0000", whatever would make it accept.