Who knows in reality - although if you're required to provide the ferry ticket it would make it pretty simple. Theres quite a bit in the link on the revenue site about requiring a customs declaration for the car to be loaded on a ferry.
From revenue.ie -
Before registering vehicles imported from GB after 31 December 2020
If you import a vehicle from GB into the State, you must complete a customs declaration. You must also pay customs duty, if applicable, and Value-Added Tax (VAT) on the import value of the vehicle. This must be done prior to importation of the vehicle. It will not be possible to register the vehicle in the State without having completed the necessary customs declaration. You can obtain more information on filing in Customs declarations.