I need to get to Lugo in Spain from Ireland?

I need to travel to Lugo in an exchange but I need to organise the travel myself. I don't know what to do- the flights to Santiago de Compostela are really expensive, and I'm not old enough to rent a car. The prices of the flights to Madrid are okay, but I need to figure out how to get to Lugo from there. I've read online that you can get an overnight train to Lugo from Madrid, but I can't find any details on the RENFE site and it's really hard to navigate. I think a train is the best option, but I'd get a bus if I have to. PLEASE SOMEONE HELP ME!!! I need to get the flights booked really soon!