Post Brexit, unless there is a better agreement negotiated, then importing a car from the UK should be no different than bringing one from, say, Japan. A used car coming from Japan is charged a customs duty at the entry point in to the EU and VRT is charged on registration. I’m not sure if VAT is also charged? I believe that VAT may have to be accounted for by a trader who sells in the normal course of their business but the VAT is charged on a margin basis rather than on selling price and input credit? If the used car is bought by a Irish trader in the UK with a VAT invoice then I believe they deal with VAT in the usual way. VAT is charged on an import of a “new” car from another EU country because thr export from that country will have been charged at 0% for export.

IIRC, the formula for imports before VRT was introduced applied an imputed credit to reduce the UK price before duty and VAT were then applied. The level of import duty was linked to the age of the car and the price paid in the UK. It was a relatively complicated formula.

VRT was introduced, in 1993, when most imports were from the UK and customs duty could no longer be charged as the free movement of goods within the EU required. IIRC, the same changes applied to used cars coming from Japan, another popular source at the time. An EU wide rate of 10% Customs Duty rate is applied to used cars when first entering the EU.

This VAT treatment before 1993 was, I believe, because at that time no business could claim an input credit for the VAT element of the purchase price of a car used in the business (e.g. company cars). Dealers selling new cars were able to claim an input credit because they were buying new stock in trade from the distributors.