Monbus has scheduled for today:
One bus for Sarria to Santiago.
Nine buses for Sarria to Lugo.
Ten buses for Lugo to Santiago.
Edit: Sorry, the above was copied from a post about going the other way. Anyway, use the link below to view the scheduling.
But you can also qualify for a compostella by walking from Lugo and save yourself two days on the busy CF. See
There's a monbus from santiago 0.00o lugo at 17.00hrs and a connection from lugo at 18.30hrs to sarria arriving in Sarria at 20.00hrs. IDEAL.