Is it possible to become a commissioned officer when I transition into the IRR, or will I need to commit to more AD time to get a commission?

Get your MHA and then speak with an amedd recruiter about direct commissioning in amedd as an officer. There are reserve and active options. You wouldusually be on the hook for 6 years of active drilling status and 2 years of IRR if you took officer commission.

OK, you can not Commission in the IRR. All the Commissioning sources are either Active Duty Army, Army Reserve or Army National Guard. The Only commissioning source that would allow you to do it on a part time basis is the Army National Guard where the Traditional OCS is two Annual Training periods about 12 to 14 months of Drill weekends. Even if you go that route, you would still need to go back on Active Duty to complete your Basic Officer Leadership Course, which will be at least 22 weeks.
All will have either an Active or Reserve Duty Obligation.

IRR is individual ready reserve. No pay for it just runs out what your contract specifies. You are pretty much a name on a roll. Very few get called up in times when needed. Given you are a health care specialist your chances are the highest. In addition you get no retirement credit in IRR. If you have your college degree you can apply for a direct commission. But you will need to still have to go back on active duty for your branch schooling. A better choice would be go reserve if you still want to accrue retirement points. But reserves do get called to active duty, when needed. Another option if you get a commission is IMA - individual mobilization augmentee. Their are slots in the army for them. They do 2 weeks every year to keep current and you can also request time to a reserve unit to get more points for a qualifying year for retirement. Hope's this helps.