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.

You'll can seek Commission towards AD, Reserve or Guard. It'll add time depending on what you sign up for. It's typically a 4 year commitment.

I've never heard of that being an option. It certainly isn't in the Navy. Ask yourself this - why would DoD commit to training you to become an officer if you were just going into the the IRR that doesn't offer them anything? They're going to require a certain amount of active and/or reserve time, and that will vary by branch. The Navy Reserve has direct commissioning programs for PAOs, engineering, intel, HR, and other specialties that enable you to go straight in the reserve world, but you have to have at least 5-7 years of professional experience in that field to be competitive.

When commissioning, you sign a new contract with active/guard/reserve time commitment (you will not e able to serve solely in the IRR).

Enlisted and commissioned time are totally separate. What is your commissioning source?

You didn't say what you wanted to do beyond being commissioned. Seeing you are a medic now, are you following on down that pathway. To be commissioned you must have qualifications for a particular job; a degree and license where applicable. But I think what your question is, can you be in the IRR (non-drilling) and get commissioned; probably not. After all, almost everyone separating from active duty goes into the IRR at least for a little while unless they have completed their military service obligation (MSO) of eight years incurred at time of entry. However many newly appointed officers are commissioned into the Reserve and assigned to units for the regular one weekend a month, two weeks annual training, and then whatever the unit deploys for.

Have you considered attending nursing school to compete a BSN via either Army Enlisted Commissioning Program (AECP), or applying for an Army ROTC scholarship for nursing.
For AECP see https://recruiting.army.mil/aecp/
Talk to you Army Career Counselor.

For Army ROTC scholarships for nursing students see https://www.goarmy.com/rotc/courses-and-colleges/programs/nursing.html

Mechanically.......... you will have to get a commission via one of the multiple sources mentioned, OCS, ROTC, Academy, Direct (etc...) Regardless of how you commission then you would need to become branch qualified which happens through your Reserve Unit. So at that point, you have started to a new commitment to the USAR, and no one is going to let you go IRR once you become branch qualified.

Technically with the right approval authority one could go IRR any time. However if you enter IRR without being branch qualified you'll get kicked out after 2 years as a 2LT. Also, it's probably impossible to convince an approving authority to transfer a new branch qualified 2LT to the IRR.

I am curious what your game plan is.

I think if you brought up this question in a Selection Board it would kill your application.