I think the dealer was trying to scare you off and knock the value down.
As far as I understand, the main difference between the versions distributed here and the UK relate to trim and extras - these shouldn't really be a concern for replacements. This is supposedly due to the "need" to keep the base cost low in Ireland because of the huge level of VRT, VAT and import duties - they need to sell a stripped down car here to make it affordable(ish) by the time all the taxes are lumped on.
I think the range of engines available in the UK is normally greater, but I didn't think there were engine variants sold here that weren't sold there - just that there were more variants in the UK.
Even so, unless the manufacturer has gone to the trouble of developing an engine specifically for Ireland there will be spare parts all over the place in Europe, so they shouldn't be a problem.
You can import the car here yourself (having owned it outside the country for a period) and avoid VRT, but I don't think there is any exemption for leaving it unused here for a period. If that was the case everyone would be sticking cars in the garage for a few months and saving 10K on their purchase.
You can sell it into Ireland alright, but the buyer will have to register it and so will factor that cost into their purchase price.
z