If you import a vehicle to Ireland from the UK within 3 years of it being exported from the EU to the UK then you may qualify for returned goods relief. This means you pay 0% customs duty instead of 10% duty. You are still required to pay VAT of 23%. A simple example of this is if you import a 2 year german car (e.g. a BMW) from the UK to Ireland 0% customs duty will apply provided it was exported from Germany less than 3 years ago. Note the relief applies to the date of export, not the date of registration in the UK.
A claim for Returned Goods Relief is included in your customs declaration. The onus is you to prove that the vehicle qualifies for the relief. The relief applies to customs duties. It does not provide relief from VAT unless the vehicle is reimported by the same economic entity as originally exported it from the EU.
You will need a customs agent to complete the customs declaration on your behalf. At CustomsDeclaration.ie we have successfully completed many customs declarations with Returned Goods Relief for people who have imported UK registered vehicles.
For more details go to Request a Customs Declaration.