I am advised by Revenue that where there is a valid invoice arising from an arms-length transaction, the value on the invoice is acceptable in determining the value of the vehicle for the purposes of calculating import VAT. The VAT arising will be charged on the purchase price inclusive of all taxes, plus the costs of transport and insurance associated with the importation of the vehicle from Great Britain to Ireland plus any customs duties or other import taxes which may arise on importation.
Details are published on Revenue’s website at https://www.revenue.ie/en/vat/goods-and-services-to-and-from-abroad/imports/when-is-vat-payable-on-importation.aspx .