When I move to Ireland this year I will have to pay VAT on my car that I am taking with me. How is the VAT of 21% calculated? Is it on the cost of the car when I bought it second hand or the cost of the car when it was new? Also as I have owned the car for more than 6 months will I need to pay the VRT?
Since you have had your car for over 6 months, you may be freed from the VRT (Vehicle Registration Tax).
Since you are transferring residence to the EU, you may also be freed from VAT and customs duties for household goods (including private cars).
Follow the links found in the first link below, which go into detail about the exact conditions.
I found no information on how the VAT is actually calculated.
For VRT there is information on how it is calculated in the first link.