GarageBand and irig is a total waste of anyone’s time. Yes, It’s completely possible to record professional quality audio on your ipad but the first thing to do is forget about garage band all together and get the right apps and hardware. Jan 18, 2017 IK is also introducing its new iRig Acoustic Stage microphone/pre-amp. An update to the previous gen iRig Acoustic, it is perfect for the stage or anywhere you need to get a “true acoustic tone. High-def jams on your iPhone, iPad or Mac/PC. The pioneers in mobile music making now bring you the new iRig HD - the next generation of the immensely popular AmpliTube iRig interface. IRig HD is the first high-quality 24-bit audio interface that fits in your pocket and connects directly to your Lightning, 30-pin and USB devices - so no matter which iPhone, iPod touch, iPad or Mac you have.

It could be too late..! But I had the exact same problem and here is how I managed to solve it:

Unplug iRIG from computer.
Plug headphones with mic that you know previously your computer recognises no problem to the iRIG.
Plug iRIG with headphones and mic to computer.
Unplug headphones from iRIG (if you want to...)

Irig Acoustic Stage

Irig Acoustic Stage Manual

Voila!... It should now have been perfectly recognised by the computer via the headphone's mic and it will now work even if you unplug the headphones from it.

Hope this solves your problem too. Good luck!

Irig Acoustic Garageband Mac