Vehicle Registration Tax (VRT) is charged at the time of registration of a car. In general, it is based on CO2 emissions at the time of manufacture and charged at a percentage of the Open Market Selling Price (OMSP). The OMSP is Revenue's opinion of the price that the car might reasonably be expected to realise, inclusive of all taxes, if sold by retail.
I am advised by Revenue that it forms its opinion of the OMSP using market information such as trade guides, advertisements, expert advice, and OMSP declarations made by local distributors in respect of their models. Revenue has published a booklet setting out the methods applied to arrive at OMSP and the VRT chargeable at http://www.revenue.ie/en/about/foi/s16/vehicle-registration-tax/vrt-manual-section-08.pdf.
Revenue provides VRT estimates for a wide range of models via the VRT calculator at www.revenue.ie and provides advice at http://www.revenue.ie/en/tax/vrt/faqs-vrt.html#question4 for customers who require estimates for unlisted models.
A person who has paid VRT, and who believes that the charge was excessive, has rights of appeal. Information on the appeal procedures is available at http://www.revenue.ie/en/tax/vrt/leaflets/vrt-appeals.html.