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.


[Feb to Aug 2021: a number of users have confirmed this procedure works; October 2021, another user has confirmed this works]
Find more info here:

Otherwise you could try connecting your iPhone/iPad to a computer and install it from iTunes.