With one purchase, you can download the in-app to all your Android devices (Android phone & tablet)
There is a "Restore purchases" button in the Store section of Settings/Styles (in older versions, More/Styles) in iReal Pro where you can download the In-apps that you purchased before.
You just need to be logged in with the email address you used for the original purchase in Google Play Store.
If you still have issues after restoring, try to reboot your device.