Drilling into Railroad apartment walls

Hello! Just moved in into an old railroad style apartment, and I'm trying to put up some shelves and a mirror. I'm used to drilling into concrete or drywall (in which case I always aim for the studs) Studfinder not really working here, so I assume it's plaster? Any of you have some tricks to finding the studs? Or do I just get some good anchors and go for it? (25lb mirror)