I'm going to do the annoying thing where I don't directly answer your specific question. BUT - I have rented a car at SDC and dropped it off in Ferrol. It was quite easy--the car rental company (Budget) had a drop-off spot right in central Ferrol. The one-way fee was modest.
If you can get an affordable transfer, that's probably simpler! If you have any interest in seeing something in the area that's off-route before heading to Ferrol, though, this could give you a little flexibility.
Welcome to the forum, @Petronie. We've used Taxiperegrino.com at various occasions when other means of transport from or to Santiago airport were inconvenient. The service had been recommended by others on the forum.
I vaguely remember that their quote was slightly cheaper than quotes from http://radiotaxicompostela.com which is the taxi association that serves Santiago airport and is listed on the airport's website and I wouldn't hesitate to use their services either.
Needless to say that it is easy to organise and use public transport from and to Santiago airport although this may depend on time of day, travelling time available and desired level of convenience.