Without a car the Ribeira Sacra is complicated
Public transport here in rural Galicia is a bit of a hit and miss. If you get stuck Taxi is often a valid and not so expensive alternative to walking. At the time of writing (12/2018) our local taxi charges 12€ to go from our casa to the railway station which is about a 10Km ride.
From all airports there is always a bus directly to the nearest town, respectively to the railway or central bus station. The exception being Porto where there is a tram directly to the town center.
A good way to start researching is using Renfe.com the Spanish railway.
Monforte or Sober do not have a car rental agency, BUT
Why not travel by train from Madrid or Barcelona to Orense? There you can easily rent a car at the station.
Return the car to the station and use the fast train to Santiago airport.
Orense to us is just 35Km and you can start your sightseeing immediately.
Contact us for more information and ideas.