Too_Sleepy I was in pretty much your situation recently. The Caa will accept an Irish Medical, but you have to do more than just send the Cert across. You must have your full medical report faxed across, and not just your cert. You will have to ask the nice people in the Matter to do this for you, to the best of my knowledge they will not hand it over to you. I would have though that just seeing your cert would be enough for the CAA, the Irish medical being JAA, but I suppose integration hasnt got as far as it should have...
The Initials are not cheap, I dont know what the exact price is in Dublin, if you call the Matter switch, and ask to get put through to the Aeromedical department they will let you know. They dont work a full week, I think they are off Mon and Tues, but they are always very prompt to respond if you leave a voicemail. I seem to remember the last time I renewed in Dublin was about 120 odd Euro.
I cant recomend strongly enough that you get a Class 1 medical Before you start your ATPL training. Plenty of people are refused a class 1, so better to spend the money before you start training. In fact I came close enough to not getting my Initial. It would be an absolute nightmare to get as far as CPL level and then be refused a medical.
The Class One lasts 1 year, until you reach the age of 40 then you must do it every 6 months.