You can install iReal Pro on an older device as long as it is in your Apple Account's purchase history. The purchase does not have to be old: you can buy iReal Pro today on a newer device signed in to the same Apple Account, and it will then be available to download on the old device.

On the old device, open the App Store and sign in with that Apple Account. Go to your purchased apps, find iReal Pro, and tap the download button. Because the current version needs a newer iOS, the App Store offers the last version compatible with your device, tap to confirm the download.

For the full steps to redownload a purchase, see Apple's guide: Redownload apps from Apple.