The first step consists of pairing the flirc with the apple remote keys on a pc using the flirc tool you downloaded from the flirc support section.
When satisfied plug the flirc back in the pi and run:
sudo /etc/init.d/triggerhappy status -l
This will show you the corresponding triggerhappy key code when you press a key on the remote.
This will allow you to add the corresponding entry to the
triggerhappy config file.
You can find some details here: Keyboard shortcuts with triggerhappy
Ignore the stuff about installing triggerhappy, it is already installed.
There also a few keycodes already preconfigured by Volumio in the triggerhappy config.
Triggerhappy source: GitHub - wertarbyte/triggerhappy: A lightweight hotkey daemon