In case you purchased the app previously, it should automatically download the last compatible version to your older device (iOS 9.3.5, iOS10, iOS12) 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.
[Latest report from users, Feb 2022: confirmation this procedure works]
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: https://support.apple.com/en-us/HT204088