An appointment to have your vehicle inspected at an NCT centre must be made within 7 days of the vehicle entering the State in order to register and pay Vehicle Registration Tax (and any other tax liabilities due on the vehicle). The National Car Testing Service NCTS administer Vehicle Registration Tax on behalf of the revenue commissioners.
You must then complete the registration process and VRT must be paid within 30 days of the vehicle entering the state. In some cases, such as when transferring residence to Ireland from another state you may be exempt from Vehicle Registration Tax provided you meet certain criteria. You will then be issued a registration number. You can then purchase number plates for your vehicle and pay your motor tax.
It should be noted that an unregistered vehicle may be detained or seized by Revenue Officials or by An Garda Siochana. Also if you do not register within the required timeframe you may be required to pay penalties.