will my mac play films striaght to apple tv over airplay or does it need a router

Here are the basics: Use Home Sharing to share iTunes content with other devices - Apple Support

You need some network connecting the two.

Suggest you trial it at home by disconnecting your internet at your modem, reboot the Mac and ATV and then try and play the movies you plan on watching.

Verify you don't run into the need to verify your purchase through an internet connection.

Also may want to consider a displayport to HDMI cable to connect Mac directly to HDMI TV.

You might be abe to use a small ethernet switch, and connect both computer and ATV to it, or even just a crossover cable. You'd need to set static IP addresses in both devices since there's obviously no DHCP server here. Now, this all relies on the Apple bonjour protocol, which might require a router to forward it's messages correctly. But it might work. As already noted, test to be sure an internet connection is not required to verify the purchase, or whatever.

If they are iTunes purchases then you will still need an Internet connection as it will need to ping the server for verification.

.