Just taking a wild guess as i know nothing about ATR, but I would suspect that the plug in the ATR could be the same as in the Airbus.
If that's the case the plug will be a "XLR 5" (5 pins) and you will need the kind of adapter comonly used in Airbus aircraft (unless one has one of those fancy Senheiser headsets that comes already with a XLR 5 plug).
Below are links that show the thing I'm talking about
: (hope that I won't get my head kicked because of publicity issues)
http://www.marvgolden.com/headsets/airbus.htm
http://www.mypilotstore.com/MyPilotStore/sep/3333
http://www.skygeek.com/airbus-adapters.html
The thing is bloody expensive. Try to search further and see if you can find a better price. If you have some knowledge of basic electrics I would sugest doing one yourself. I bought the material and made one myself for about 10 dollars.
Cheers,
Zero