There is still, quite bizarrely, a lot of confusion here on the Big Island about what is in the UK and what isn't. It is by no means unusual to see people get off the Dublin ferry with Sterling cash and be quite agitated when it's refused. I have posted stuff to Laois on occasion and have been told, more than once, by Post Office counter clerks, that "it must be in Northern Ireland", I've had to argue to get the right postage on it and avoid the issue at the other end. I have even had that argument with the same clerk twice now.
People are also often very perturbed by the lack of a Postcode, even though Eircodes are available now - if you do use an Eircode, you risk being told that it is wrong, because it 'looks different'.