The trick is that you have to already own an iReal Pro app in order to download an earlier version.

If you don't already own iReal Pro, purchase the iPhone/iPad app for 7.1 through the iTunes store on your desktop or laptop computer.

Then when you go to download on your earlier version of your iPod, iPhone, or iPad, you'll be given the option of downloading the earlier, compatible version of iReal Pro.

You must use your same Apple ID on the computer and iOS device for this to work.