Do you mean you want to get to Ponferrada on the evening of the day you fly into Santiago? If so, you'll be pushing it a bit but it's possible if you don't mind arriving quite late. Just make sure you don't try to walk too far the next day! As well as the buses mentioned above, you can get a train, leaving Santiago at 6.35pm with a change at Ourense, and arriving at Ponferrada at 10.19pm. I think that might be your best option - the first bus time available to you would be the 21.30 one on the timetable above (the one at 17.00 doesn't run on the day you need), which would get you to Ponferrada in the early hours of the morning.
For accommodation in O Cebreiro - we stayed at a casa rural called Venta Celta, after it was recommended on this forum. We had a double room with private bathroom for 40 Euros and it was fine - cosy and clean, and the owners were very nice and helpful (and spoke excellent English). There is a big bar/restaurant downstairs where you can eat. You can book by email at
ventacelta@hotmail.com.
Buen Camino!