Yes, of course! If you want to book a car at the Fishguard Ferry Port, you must set your pick up time between 11:00 and 13:00 hours from Monday to Friday. This location is closed on Saturday and Sunday. This location is only open during the arrival of the morning ferry (arrival 11:15 hrs).
You must chose a return time in normal office hours when booking online. However, using the key drop box it is possible to return the car at anytime.
The office is not open for the arrival of the evening ferry.
Another nearby option is Aberystwyth, UK which normally has vehicles at normal office hours. The location is just outside the Ferry Terminal.