I always write 'EIRE' in the centre of the top of the envelope for post to state departments.
All government departments have Freepost licences so posting without a stamp is okay as it will be paid for at the delivery end.
I think it's use stemmed from the ''harp'' symbol that appears on passports & appears on the top middle of government issued envelopes.I've never heard of this before. Is there some background to doing this? I always use "Freepost" on first line of the address.
This is why I love AAM. Where else would you find it.