We will be traveling around Ireland and then heading over to England for a couple of weeks. We are planning on leaving our rental car at the airport and getting to the ferry terminal. Is there a shuttle from the airport or should we take a cab? If so what do cabs normally cost to get to the ferry? We will be renting a car on the other side in England.
Why not just fly to your destination seeing as you'll be at the airport already?
Taxi to the ferryport would be around €20-30.
You mention hiring a car in England, how do you plan to get there from Holyhead?
Maybe FFS's question is really, "To where do you plan to drive from Holyhead?" Somewhere near another airport? There is car rental near the port in Holyhead.