Before you enable iCloud Sync, make a Backup (iOS) of your library on all of your devices (for Mac Backup, click here)
To enable iCloud Sync you need to login to your iCloud account in the Settings on your device and enable iCloud Drive.
iOS: enable iCloud Sync in iReal Pro's Settings (tap cog wheel icon at the top of the Song Library window; or in the More window on older versions);
Mac: iReal Pro > Settings (Preferences) > iCloud sync, click Sync
Allow some time for the first sync depending on your internet speed and the size of your library.
If you plan to enable sync on multiple devices, choose one that has the most up to date library and enable iCloud Sync. We then recommend deleting all content from the other devices before enabling iCloud Sync. If you do not do this, enabling iCloud Sync will merge the data on your device with the data in iCloud which can sometimes cause duplicates that will need to be removed manually.
Always allow some time for changes to sync between devices.