My understanding is that it may cost 25c to get iMessage activated because it requires iTunes to send a UK text as part of the process. Once activated it should not cost you anything but note that some people have reported needing to do the activation again when they go abroad and then return to Ireland. See here and there are other threads/posts dealing with iPhone/iMessage/FaceTime activation issues and costs elsewhere that might be worth searching for.
iMessage & FaceTime Activation
Hope this helps.
