Mac Pro's dont have quicksync that is required for airplay mirroring. Maybe the next gens will get it. You are better off using arparrot. Sorry to disappoint.
You can't. No Mac Pro is compatible with AirPlay mirroring. The next rev of the MP should be though. It's because of the hardware encryption that the sandy bridge/ivy bridge processors have.
Mac Pro's may not have Quick Sync in their Xeon processors, however they are more than capable of handling it based on their CPU power alone.
I've been using "AirParrot", a third party app that does AirPlay/Mirroring and more. I have a 12-Core 5,1 Mac Pro with an ATI 5770, using iStat to monitor CPU load, AirParrot does a fantastic job. Interestingly, using OS X's native AirPlay on my current gen 27" iMac taxes the CPU more than AirParrot on my Mac Pro.