Hello
I claimed back tax on braces some years ago.
As you suggest, u simply get the dentist to complete the Med 2 form and submit this as part of your tax return.
At the time I did it I waited until the end because it made sense to do so - at the time there was the €125 threshold to factor in. That threshold no longer applies so there is no need to wait.
Suggest you have a word with the dentist and say u plan to claim back on a tax year by tax year basis. There shouldn't be a problem as he/she must encounter this issue on an on-going basis with other patients.
No harm to keep the receipts - although the Med 2 form should cover u (as it is signed by the dentist and effectively acts as a consolidated receipt)
Cheers