If she meets the criteria for 'dependant' on the [broken link removed], then yes, you will get tax relief for these expenses when you submit the [broken link removed] signed by your dentist.
One further small tip — regardless of when you pay for the actual treatment (typically, your orthodontist will have requested €1.Xk up front, then another €1K, then another final €1K, towards the end of the visits) — try to claim tax relief on the full amount in a single year, presumably after the end of 2006. That way you won't have to deduct the applicable "excess" of €125/€250 each time...
By the way, I may be wrong, but your query suggests you haven't previously claimed tax relief on medical expenses..(?) — like many taxpayers, apparently. You should really look into it..!