Any JAA member state who follows the JAR-FCL rules is able to let you have a medical where you want to do it. Like in your case a easy task.
That is not always the case before you have your pilot's licence issued. Once the pilot's licence is issued, the associated medical can be renewed in any JAA state recommended for mutual acceptance regarding JAR-FCL 3. However, the Irish and UK CAA seem to cooperate quite nicely (as opposed to other European aviation authorities) so hopefully you'll be ok. Just ask and you will find out if you have to go for a trip to the UK.