How will my not attending AT in the final year of my Army Reserves contract impact going into the IRR?

Possible. Since the IRR packet requires Commander signature. Also, since it's September when you're able to transfer to IRR and AT is in June.....unless you have a damn good reason to convince your Commander.....pack the bags because you're going to AT.

AT is several months prior to your ending USAR. If you don't go to AT then you have not met the requirements for the year. Your commander can based on that refuse any attempts to go to the IRR, until your USAR requirements have been met.

The best way to go about this is to speak to your CoC. But, you occupy a position on a team. Others are counting on you. Your presence and attitude at AT matters. As simplistic as it sounds, you made a commitment. Unless there is a really good excuse to miss AT, you need to be there. For yourself and for your team. Simply wanting to or not wanting to is irrelevant.