iReal Pro version 8.3 or higher:
If you record with GarageBand or similar apps, export your iReal Pro backing tracks in WAV format.
Then select 'Copy' from the share sheet options and paste it into a track in GarageBand.

iReal Pro version 8.2.4 or lower:
There are 2 other solutions as well:
1. Record directly from iReal Pro. Open GarageBand, add a new track and select an Inter-App
Audio app. Open the Instrument browser and tap select iReal Pro. 
2. You can record any track directly from iReal Pro into GarageBand with Audiobus and use
effects.

You find a tutorial here how to record the sound directly into Garageband.