The deduction of the V.A.T from the gross purchase price only applies to new houses, as Eamon66 rightly says. However, the stamp duty computation on a new house for an owner occupier (whether FTP or not) is not based on this anyway. Briefly, for a first time buyer (and owner occupier) of a new house the situation is as follows:
1 If the house is "grant sized" (125 sq. mtr. gross internal floor area), and there is a "floor area compliance certificate" there is no stamp duty. EXCEPTION: stage payment houses, where the transfer deed is signed before the certificate issues. (a problem about which the Law Society are lobbying).
2. If the house is over grant size, it is exempt from stamp duty unless the gross price goes over €1,441,450.00 OR the site price as a separate item (but as part of a house-plus-site transaction) goes over a gross price of €360,362.50.
For an owner occupier who is not a FTB, the corresponding figures are €576,580 for house or €144,145 for site as separate item.
For a second hand house, gross price is all that counts.
If there is a civil service prize for drafting taxing statutes with needless complexity, the boys in the stamp duty section must win it every year.