Is it possible to have postcode only show for certain countries?
We're using 4.1.4 and 4.2.4
Ireland has no postcode system, so we'd like it not to show for Ireland,
However, UK / USA should show and require this,
Is this possible out of the box with CS-CART?
Or is there a solution / addon available?
Not out of the box. It requires that a hook be inserted into one of the core files to allow you to dynamically set the profile fields based on the localization in place. But to do this, you also need to enable localizations which can be a pain because they are really only partially implemented. Note that things like profiles, blocks, etc. have variations based on language. But localizations can share a language and may require different content (like you've discovered). Same with blocks that may specify currency and/or imagery that is location (or language) sensitive.