Santiago Airport to Villafranca

There probably are other buses as the smaller companies don't post their services on-line, but they'll be infrequent anyway. That isn't a lot of help to you I know, and given that you are travelling tomorrow why not just enjoy Lugo for a few hours? The bus station is right by the walls of the old city, you can easily spend an interesting couple of hours in the old town without walking too far, it's one of the best placed bus stations I've ever seen, and the weather should be warm and sunny tomorrow, there's wall to wall sunshine today.
Buen Camino
Sue