Ok so the offical response from the revenue is as follows:
Section 132, Finance Act, 1992, provides that VRT shall be charged on a category A vehicle at the rate of a percentage of the value of the vehicle. Section 133 of the Act provides that the value shall be taken to the open market selling price ("OMSP") of the vehicle at the time of the charging of the tax thereon.
The section provides that the OMSP of a new vehicle on sale in the State, which is supplied by the manufacturer or sole wholesole distributor, is the price declared to the Revenue Commissioners, inclusive of all taxes and duties, which, in the opinion of the manufacturer or distributor, a vehicle of that model and specification, including any enhancements or accessories fitted or attached thereto or supplied therewith by such distributor, might reasonably be expected to fetch on a first arm's length sale thereof in the State by retail. Thus two new cars of the same make, model, version, and specification, supplied by the sole wholesale distributor, registered on the same day, will attract the same amount of VRT.
The section provides that, in the case of any other new vehicle, the OMSP is the price, inclusive of all taxes and duties which, in the opinion of the Revenue Commissioners, would be determined under the section if the vehicle were on sale in the State following supply by a manufacturer or sole wholesale distributor in the State. Thus a new car, of the same make, model, version, and specification, as the cars mentioned above, registered on the same day, but supplied other than by the manufacturer or sole wholesale distributor, would attract the same amount of VRT.
The section provides that, in the case of a category A vehicle other than a new vehicle, the OMSP is the price, inclusive of all taxes and duties, which, in the opinion of the Revenue Commissioners, the vehicle might reasonably be expected to fetch on a first arm's length sale thereof in the State by retail, such price to include the value of any enhancements or accessories which, at the time of registration are not fitted or attached or sold therewith but which would normally be expected to be, unless it is shown to the satisfaction of the Revenue Commissioners that they have not been removed or not sold therewith for the purpose of reducing its OMSP.
The Revenue Commissioners determines the OMSPs for vehicles other than new vehicles by methods including reference to trade guides, advertisements, and consultations with the trade and other knowledgeable persons.
And in response to a subsequent letter asking for clarification that 'all taxes' includes VRT:
The expression "all taxes" does includes VRT. VRT is charged once only on registration of the relevant vehicle.