At the moment there is no en_IE because there hasn’t been much (i.e. any to my knowledge) demand for it and I don’t think it would make a big difference anyway. What I gather the language and locale data differences to en_GB should be fairly small if nearly non-existent on phone context. Currency is different but that shouldn’t be used from the locale, and it’s not really used on the UI anyway that much.
For contributions, the locale data should already exist for this and that on lower level Qt side. The problem with translations here is that I don’t think it makes sense to have a fully separate translation for more English variants but rather have a fallback system so the variants would only contain the delta where needed, if any. We have now translations as “en_GB” and “en_US”. If we renamed former translations as generic “en” then we could be relatively easily add some variants, but then we couldn’t be adding variants of en_US.
Not totally out of the question, and in the long run it would be nice to have better support for the locale variants, but I’m not yet entirely convinced if there are enough use cases where this should really matter.