If your NAS offers iOS apps to access its files or has an iTunes server built into it (like the Synology, and probably others), the options are:
1) With iTunes Server, you should be able to access the library from any iTunes client, which includes your iOS device, if you set it up correctly on the NAS. You can't sync or create playlists this way, but it's accessible. AirPlay gets you to the AppleTV.
2) An app that lets you browse the NAS on your iOS device will most likely launch the appropriate player for media files (The DS FIle app for Synology does this). Again, AirPlay needs to be used for the AppleTV.