I am not sure which option is the fastest, but here are some ideas on how this could be done.
1)
http://www.alsa.es (bus) from Madrid to Galicia.
The bus passes the Cebreiro on the "interstate" but I am not sure if it makes a stop there. If it does, this options would be a good one.
2) Fly to Santiago and then bus "back" to Cebreiro. Not ideal, but.. there used to be a bus last summer direct from Santiago (busstation) to Cebreiro that left around 17-18 or so.. not sure if it runs this year. Alternative is taking the bus to Lugo and then another bus from there... with this option you would probably need another day.
-- Taxi from Santiago airport to Cebreiro is about 120-140 euro if I remember correctly...
http://www.iberia.es has several good deals from Madrid to Santiago (about 50-60 euro usually).
3) Train from Madrid and bus to Cebreiro (look for schedules and the best station to get off on),
http://www.renfe.es is the train site.
I know this list is missing detailed information, but I hope it gives you a starting point for finding "your way". Maybe someone else has some other alternatives?
Saludos from sunny,
Ivar