Pre-Paid Royal Mail Envelopes from Eíre

[Newbie!]

Quick question. I need to return something to a store in the UK and there is a pre-paid Royal Mail sticker which I need to fix to my parcel when returning. Just before I posted it yest, I checked the stores website and it said this pre-paid sticker only works in the UK and N.Ireland.

Does anyone know what would have happened, had I just posted it as normal yesterday??

 

[BillK]

Presumably, if it was delivered than the recipient would be charged; assuming that it would be delivered with less than the required postage paid.
(I assume that this would be the case as posting letters to Ireland from England is approximately 50% dearer than posting within England to a UK address.