Hi, philro,
I haven't made this trip either, but I have pretty good luck using the website movelia.es to find bus routes. Looking there, I saw that you can either go via Ponferrada or via Bilbao.
Option 1 -- SS - Ponferrada, leaves 9 am, arrives 6:30 pm (41 euros)
Ponferrada - El Ferrol, leaves 6:45 pm, arrives 9:30 pm (22 euros)
Option 2 -- SS-Bilbao (many buses, one leaving at 9:10 am gets you to Bilbao for the one bus daily to El Ferrol) (6 euros)
Bilbao - El Ferrol, leaves 1045 am, arrives 9:30 p.m (22 euros)
I always have a much harder time finding connection information on the RENFE website (trains) and I haven't been able to find an easy route, but there must be something. I can see there is an overnight train from Madrid to El Ferrol, so there must be a way to connect with it from the east.
And as JohnnieWalker says, there is the FEVE line, a narrow gauge train that runs along the coast. You can start on it in Santander, and then change in Oviedo. Getting to Santander from San Sebastián is easy, there are many buses each day. I would think the FEVE is a slow, but scenic option. Pricing from Santander to El Ferrol seems really cheap. FEVE has its own website, since it's a different train system completely.
http://www.feve.es/es
But as Johnnie says, I'm sure there's someone with actual knowledge who can sort this all out.
Good luck with this.