How does IRR drill work?

There are different control groups in the IRR, one of which allows for annual training.
The idea of the Individual Ready Reserve is to keep you qualified so that you are ready to deploy, if needed, in a short amount of time.
If you are placed in that control group HRC would be publishing your orders so you would have to be in regular contact. You're also supposed to be able to be promoted but I'm not sure how that works.
If you do go that route, ask HRC how you would receive your AT order because your military email will now than likely be deprovisioned, which makes no sense. You may also lose your CAC, making even less sense.

While I was in the IRR I had no contact with anyone. I received orders once for an SRP type event. I was on ADOS tour so I maintained my CAC, email, and MEDPROs.
My friend transfer to IRR and when her CAC expires (while in IRR) no one would renew it, so she was cut off from all military access.
IRR maintains service, but you don’t typically have good years toward retirement. There are ways to drill or do EBDL but it is hard to find a POC at HRC that will work with you.
My friend and I both got passed over for promotion in our primary zone while in IRR. I was told it didn’t matter (being IRR) and I’d still be competitive but I don’t think that is true.
You are not a priority for anything while in IRR.

Do you mean, transferring into a unit to drill? Or gain point while in the IRR? IRR Soldiers can take part in IDT when authorized by HRC. The steps are laid out in the IRR handbook. You have to find a unit to be attached and submit a DA form 4651 to HRC requesting orders. If you would like a copy of the handbook I can send it to you, or you should be able to get it from the HRC website under the IRR section. Are you collecting VA? Or what concerns do you have joining a unit?