Just to clarify, in the first post PRSI cover is mentioned, which would be Dental Benefit for regular dental work. This only covers €85 for root canal, so just a tiny fraction of the cost.
What Clubman has linked to is for a refund of PAYE tax, where you will get back tax on what you pay out.
Apologies for any confusion caused. As Berni says I was referring to tax relief and not the PRSI cover for the treatment in question. The details of the latter are available on www.welfare.ie. Anything that is not otherwise refunded or covered should qualify for tax relief.