Least hassle given your landing time would be to find some accommodation close to the intermodal (bus) station in Santiago and catch the direct bus to Sarria the following day. You may find others on your flight or milling about the airport who would be willing to share a taxi but no guarantee.
There buses to Lugo from the airport at 17:15, 18:45 and 20:15. You may be able to pick up a connection to Sarria or a less expensive taxi from there.
Compra online billetes de bus y consulta horarios. Si lo prefieres alquila un vehículo para tu viaje en grupo. Viaja seguro, confortable y barato con Monbus.
www.monbus.es