Transport between Villafranca del Bierzo and Tricastela

Alsa company between Villafranca and Piedrafita do Cebreiro .
Monbus http://www.monbus.es between Piedrafita do Cebreiro and Triacastela; not in Sundays, just one run, leaves Triacastela 18:43; leaves Piedrafita very early (06:45). Or taxi. Or walk.

The Monbus bus operates from Sarria to Piedrafita do Cebreiro once in each direction. Lv Piedrafita in the AM and Sarria in the PM.

So, depending on your schedule ... it might work out better to take the ALSA bus from Villafranca (or Ponferrada where it originates) to Lugo, Monbus from Lugo to Sarria, and Monbus from Sarria to Triacastela.

The other bus company is Autobuses Gonzalez de la Riva. They have a schedule from Ponferrada to Vega de Valcarce (with stop at Villafranca). You can arrange (in advance only) to go from Vega de la Valcarce to las Herrias and la Faba ... close to but not quite O Cebreiro.
http://gonzalezdelariva.es/index.html

Note ALSA calls it Pedrafita do Cebreiro while Monbus calls it Piedrafita do Cebreiro. Accept no substitutes; there are villages with similar names in the area.

Piedrafita is 4 km from O Cebreiro, that is, the intersection with the usual Camino and the albergue.
Taxis in Piedrafita do Cebreiro, http://www.concellodepedrafitadocebreiro.es/concello/taxis.php
Upon the rome2rio webpage, cost could be 30-40 euros (but don't trust this too much...)
If you are in Triacastela, ask in the Concello (city hall); its webpage does not work. Or hospitaleros, who always are helpful.