If we take the ferry from Belfast to Scotland, is there a car rental company at the ferry terminal? We want to spend 3 days touring Scotland before flying out from Glasgow.
In a word no. There used to be, but not now.
The usual advice is to come over on the Hannons or Citylink coach to Glasgow, and hire a car from there.
The nearest option to Cairnryan that we know of (you would need to take the Citylink bus, not Hannons) is at Ayr. I see there is an Enterprise branch there, and a local company called Ayrshire Vehicle Rental.
I don't know that anyone on here can review AVR but they have 5 star reviews on facebook.
Arnold Clark also have an office at Ayr.
Prestwick Airport (just north of Ayr) also has Celtic Legend. Everyone on this forum who has used Celtic Legend gives rave reviews about them.
I am sure some people will chime in, and tell you to fly to Glasgow instead, but to me you have asked about the ferry option.
Equally I am sure you appreciate that ferries can be cancelled or delayed in severe weather, as can aircraft.
If you only have three days for Scotland, I recommend flying. The ferry will take a lot of time. If you land at Glasgow airport, there's a full range of car rental companies right there. One tip: ask the agent which signs to follow at the roundabouts leaving the airport campus to get onto the Motorway in the direction you want.