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

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