I found this on another website. I'm not sure how accurate it is, but with so many people starting in Sarria/Samos I am sure you can get there, even if you have to hire a taxi.
About travelling from Santiago to Sarria, yes, you need about three hours because you have to wait between one and other. You have to take Freire bus from Santiago to Lugo. And then, from Lugo to Sarria, you have to go with Monbús bus. Here there are the phone numbers you may need:
· Santiago's bus station: 981 542 416
· Lugo's bus station: 982 223 985
· Freire buses enterprise: 981 588 111
· Monbús buses enterprise: 982 254 040
· Sarria's bus station: 982 532 620
(I edited out my suggestion to take a taxi after Sil's post. I then found a post that said the taxis charge around 1 Euro per kilometer. That's good info to file away. Thanks Sil.)