AFAIK a JAR medical is a JAR medical and should be valid with either your UK issued JAA licence or your Irish issued JAA licence.... BUT... there is a stipulation in the regs that the medical must be done in the country of initial training.
So if you are embarking on a flight training course, you will need to do the medical in Dublin.
Look at
www.jaa.nl for JAR-OPS 3
Mutt