Well, actually, you should view your site as requiring “multilingual” capabilities, even if the base language for both countries is identical, because the actual locale for each is nevertheless different.
More specifically, you can create variant “translations” for your British (‘en_GB’) and Irish (‘en_IE’) localizations. A plugin such WPML (WordPress Multilingual) can help you implement this, and it supports different URL schemes for differentiating between the localizations, including:
- different domains (or subdomains)
- different subdirectories