We went to Cork (Cobh) on a cruise a couple of years ago and I arranged to rent a car for the day. We were met at the port gate at an agreed time (by a guy who said his name was Paddy......) and taken to a nearby car rental place. This gave us the option to go into Cork, but also do some other exploring.
First thing we drove to Kinsale and took a guided walk around that delightful town from the Tourist office, then we set off and explored the countryside and a few beauty spots before going into Cork, parking and having a wander around. Back to the car rental place where they took us straight back to the ship in time for sailing.
I can't remember the name of the car rental place, but suggest doing a Google search on Cobh / car rental.
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P&O Caribbean, New England, Norwegian and Mediterranean cruises.