As Trucker says, there is a carpark by the port but it is now on the left as you drive out of the port, it is smaller than the old one on the right which they seem to use for staff parking now. We stayed there last year as we were on the early morning ferry, it was ok and fairly quiet but it can get quite full.
We have also used Camping Trologot about 3 miles away at St Pol de Leon. They have a small grassy area (6 or 8 pitches with hook ups) at the front of the site especially for late or early arrivals, water is at the main washing/loo block. It's on a quiet road next to the sea and you can see the ferries come in across the bay. English is spoken and you can book by email, we'd use it again when sailing to Roscoff.