Driving Ireland car rentals across the Irish - Northern Irish border is generally okay. Search for any information telling you otherwise in your car hire contract and tell your rental company your plans to visit Northern Ireland. Driving your Ireland rental car across the Irish Sea via a ferry to Wales or England is a different matter. You will likely need European travel insurance coverage and pay a cross-border fee before getting your rental agreement. One-way journeys are a no-go as you will need to return your car hire to the same car rental desk. If you're not sure, call or email the Ireland car rental agency to double-check.