Galicia, Spain - transportation and hostel questions!

The value of a Eurail pass depends on how many trips you're making, and how far. Unless you're using the train a lot, and making long journeys, it's usually more effective, cost wise, to purchase point to point tickets.
For your journey to San Sebastian, there are several trains/day that make the run into Hendaye, on the border with Spain. Just outside the train station there is the little EuskoTren station which will take you into San Sebastian. Booking the first leg at tgv-europe.com will cost you about 30 Euro per adult...some advance discount rates will be as low as 25 Euro. From Hendaye to San Sebastian is a couple of Euro each, and takes about 40 minutes. Some of those French trains will also go on to Irun, just across the Spanish border. There, you can transfer to the Spanish RENFE trains, which make a quicker run to San Sebastian (about 20 minutes) but cost a couple of Euro more.
Where are you going in Galicia? If you're heading for Santiago or A Coruna, there is one train/day from San Sebastian that goes there...travel time is about 11 hours, and the cost us under 29 Euro per person. I would reserve ahead at www.renfe.es to make sure you have tickets on the day you want. The site is in SPanish, but we can help guide you through it if necessary.
If you're going to Vigo, there is also one daily train that takes about 11 hours to cross northern Spain. But it requires a transfer at Ourense, and there's only 5 minutes allowed for that...too close for me. It's also a fair bit more expensive, for some reason.
The bus service in Spain is excellent....but you won't find better deals in terms of price or timing than the train.

Bottom line...if those are the only two train journeys in Spain, a Eurail pass isn't necessary.

Thank you so much for your great information and advice!!