Will my IRR (submitted) packet get denied if I fail a PT test?

You realize that a PT failure can get you flagged. That means no personnel actions, except ETS. It can freeze you out of the IRR slot. So damn well don't fail.

Have you spoken with your Retention NCO as they are to be included in the process?

As someone else already mentioned, Soldiers can transfer to the IRR with a transferable flag. The unit can deny the request, but it depends on various circumstances. Some of these include (but not limited to) the Service Member has completed his/her SELRES obligation, has hardship, pregnancy, moving for overseas employment, missionary work, cogent reasons, etc.. You should review the USAR Policy for this (Reassignment of AR TPU Soldiers to the IRR). It is located on the USAR Personnel Actions Guide (PAG), https://xtranet/usarc/usarpag/
Having a flag does not "freeze you out of an IRR slot". There are an abundance of Service Members in the IRR who have flags for or do not meet standards for Army Physical Fitness or Army Body Composition. Just keep in mind that entering the IRR with a flag can hurt you later on, if you decide to serve again in a unit. This is because a unit Commander will have to approve the request to be placed into their unit. Life changes all the time so you may not care now, but next year you might decide that you want to go back into the Army Reserve. It is rare for Commanders to accept IRR Soldiers with flags. The Commanders typically have a lot of issues with their current Soldiers so they are very reluctant to accept more problems.
Have you considered transferring to another unit? Why do you want to go into the IRR?