The Revenue medical/dental expenses relief leaflet and Med1/Med2 forms listed [broken link removed] cover this. I think that they say that you submit your claim after the end of the relevant tax year (at which stage you will have your P60 which might also be useful). In my experience they refund any tax overpaid rather than adjusting your credits.
Your form needs to be accompanied by your p60 which means you will have to wait untill next Jan/Feb to claim. You will get a refund cheque and a new slip indicating your tax paid for the year etc which will replace your p60 slip if you need it again.