I suppose different brands have different warranty conditions......what brand are you purchasing..... I know in some warranty work can be carried out in NI or Southern Ireland. However, you need to be aware that some cars have different warranties, eg, could have a 2 year warranty in ROI but will have a 3 year warranty in UK.
In that case, if anything goes wrong during third year of manufacture, you will have to have the work carried out in NI/UK (possibly at place of purchase, not sure if it they are that strict)