So you're using an external keyboard which is ALSO hooked up to the iRig, and the messages are not getting through from this external keyboard to the MKS50, right? This is called MIDI Thru. We don't currently support it, but: if you attach the devices in the right order, you should get MIDI thru happening anyway.
Assuming I've understood the question correctly, let me know if you have any success experimenting. If not, you might have to wait until we implement MIDI thru. Thanks!