I've seen a few, alright. It used to be my job to see them, so I was paying particular attention. Can't think of any precise locations, though, I'm afraid.
There is a good variety of ciphers/insignias/callthemwhatyouwant on postboxes- one of the rarer ones is S.E. for Saorstat Eireann. There's a wall-mounted box in Monaghan town bearing this lettering.
My favourite boxes in the country, though, are the hexagonal pillar postboxes from the Victorian era- same lettering, but smaller and far more decorative than the standard pillar postboxes. Acorn finials, beading around the base of the cap, etc. I've heard there are 6 in the whole country, of which I know the locations of four. It's a bit of a personal quest.
PS I think there was a thread a while back (a good while, now) where this was discussed in a bit of detail. I'm not having a go- just saying that if you wanted to do a search (I haven't the time right now) you might find more.