My old Doc used to issue CAA medicals and simply cross out CAA and put IAA there stating he has run out of the irish, this has not been mentioned nor does it seem to be a problem?
You could always email your co-ordinator to check though?
Medical also needs to be valid 3 months after the date of joining so if you are due to renew prior to joining then ensure you get an irish one from Doc Orton just to be on the safe side.
YYZ