1

I don't know if there's public transport to San Andrés de Teixido (the tradition is to go there walking). The closest train station is like 15kms away. Otherwise, the rest of your route can be made by public transport although you should have to plan some legs carefully.

San Sebastián-Santander by bus. Bus company: Alsa
Santander-Potes-Fuente De by bus. Bus company: Autobuses Palomera
Fuente De (or Potes)-Unquera by bus. Bus company: Autobuses Palomera.
Unquera-Cudillero (or Luarca) by train. Train company: FEVE
Cudillero-Ponte Mera (closest train station to S. Andrés de Teixido) by train. Train Company: FEVE. If you plan to take a taxi to S. Andrés de Teixido, it would be better to get off the train in Ortigueira.
Ortigueira-Ferrol by bus: Bus company: Arriva.
Ferrol-Santiago de Compostela by bus. Bus company: Monbús
Santiago de Compostela-Santander by bus. Bus company: Alsa.

2

You're my hero ;) Thanks very much for your help!

3

I was going to write something smart but then anillos wrote something way better. Impressive Anillos !!

4

You can also go from Ferrol to Santiago by train, first from Ferrol to A Coruña and then A Coruña-Santiago. However, it'll take you probably three times as long as going by bus.
I have been visiting San Andrés a number of times, but have never seen a bus and agree with #1, that you would have to take a taxi from Ortigueira or walk. In case of walking, you should try to go to Cariño, which is big enough to have regular bus-services and start walking from there. Another possibility would be to go by bus to Cedeira and walk from there. I don't think there is much difference in distance between the two. You'll have to go up the mountains, so count on at least a two hour walk.

5

The bus company from Ortigueira to Cariño is Arriva. Arriva has also buses from Cariño to Ferrol (you'll have to change buses somewhere along the route).

BTW, although the web of FEVE will tell you it, I forgot to say that from Unquera to Cudillero (or Luarca), you have to change trains in Oviedo.