Will using Airplay with local files sent to AppleTV (2nd Gen) count towards data allowance from ISP?
AirPlay is a peer to peer networking protocol, so it works even if you have no internet connection. In most cases, you don't even need WiFi for AirPlay to work so if you're really concerned, you could shut down WiFi or unplug it from the router entirely and check that things work.

AirPlay doesn't disable any background downloads or connections, so you could still use data while you AirPlay, but it's not because of AirPlay

Good discussion of AirPlay and LTE/Cellular data is on this thread, but your question is different in that it's more about WAN/LAN than a cellular link:


Can I use 4G LTE (Cellular Data) and Airplay using Wifi w/ No Internet connection at the same time?


It does reinforce the "downloads can happen" aspect as well, but hopefully makes it clear the AirPlay is purely peer to peer and not routed traffic out a gateway be it WiFi or LTE.