No they wouldn't they would have a right VRT amount. That's the point. The OMSP is calculated incorrectly. 20-30% of the OMSP is VRT so therefore the VRT is 20-30% more than it should be.
You are right - the way they do the calculation doesn't make sense; they are comparing apples to oranges. A UK reg car is not worth the same as an equivalent Irish reg car because the former has a VRT liability. If you went to a forecourt and looked at two secondhand cars which were exactly equivalent in every respect except one had a UK reg, would you pay the same for UK one? No you wouldn't - you'd offer much less because you'd factor in the need to pay VRT. Therefore the OMSP for an Irish reg car is simply NOT an appropriate guide to the value of an non-Irish reged car.
Given that you'd never, for example, be able to sell a UK reg car with an OMSP of 12K for that amount on the open market in Ireland, then it is hardly an "Open Market Selling Price". If 12K was a fair price for the equivalent with an Irish reg, you'd only get 9230 for the non-Irish reged car (assuming 30% VRT applied).