Receiving post from Ireland via Royal Mail (?)

HMC

Registered User
Messages
199
I have recently received correspondence from two Republic of Ireland addresses but with Royal Mail postage. How does that work? Just curious...
 
They post them from the UK (NI?) to cut costs perhaps?
Or maybe Royal Mail offer such a service to individuals and/or businesses in the RoI?
 
As above and certain Irish-based businesses, subsidiaries of UK operations, generate all their correspondence in the UK and post it here at UK rates generating huge savings.