Remember though that, for makes including Audi, BMW, Mercedes, Lexus (maybe others, I'm sure fellow posters will oblige), the VRO will request the chassis number so that they can do a check for extras on the car (on which you pay VRT on the Irish price, natch). I think you get a cert. for the Garda in these circumstances. So there may be a delay in getting the exact VRT amount you have to pay in that case.
If a car is fitted with a pack - e.g. communications pack - which was often fitted at a discounted price in the UK then you will pay VRT on the Irish pack price or, if there is no equivalent, on the separate options at the retail price.
I'm sure somebody on here got caught by this a while back.
SSE