You need an AirPlay compatible device connected to the same network as your Mac like an Apple TV or AirPort Express, or any supported AirPlay device.
Here you find more info from Apple:

With Mac OS X 10.8 and above:
Option click on the Audio icon in the menu bar and choose your Audio device (speakers) as the Output Device or click on the Audio button inside iReal Pro in the playback control to set up the audio output.
If you have an Apple TV, you can alternatively turn on AirPlay Mirroring to mirror both audio and video to your TV.

With Mac OS X 10.7 and below:
You'll need a third party app like Airfoil to send the audio of iReal Pro to your AirPlay compatible device. Once you have that setup you'll notice a 2 sec. delay of the sound between iReal Pro and the AirPlay device.
Fix this in the Preferences of iReal Pro by checking the AirPlay offset option.