In case you purchased the app previously, it should automatically download the last compatible version to your older device (iOS 9.3.5, iOS10 to iOS14) if you previously purchased the app.
If you have problems, install the Last Supported Version from the “Purchased” section.
Tap on Purchased, look for iReal Pro among your purchased apps, then the download button.
[June 2024: confirmation from our users with older iOS devices, this procedure works without problems]
Find more info here:
Otherwise you could try connecting your iPhone/iPad to a computer and install it from iTunes.
This method only works if you have previously purchased iReal Pro from the iOS App Store. Please make sure you are signed in to the Apple account you used to purchase iReal Pro originally.
Apple purchase history: