1) So, IRR time is the time that you are on reserve during med school, correct?
You are technically in the reserves during medical school. When you do the HPSP, you sign an eight year contract, which includes active-duty service obligation (ADSO) and inactive ready reserve (IRR) time. Whatever time you aren't doing ADSO, you spend in IRR. Let's say you have a four year ADSO. After you serve four years in active duty, you spend the rest of the eight year contract in IRR, unless you do an active-duty residency, which counts toward IRR time. So let's say that you have a four year ADSO and do a four year active duty residency; that amounts to zero IRR, as far as I understand it. However, if you have a four year ADSO, but do a civilian residency, you are going to owe four years in IRR.
There is a chance that you could get mobilized while in IRR, but the way that the people I talked put it was that it would have to be WWIII for it happen. I've never heard of any doctors being deployed from IRR.
2) If I have a 3 year scholarship, I only need to do 3 years of ADT. But, if my military residency is let's say 5 years, then I would have to owe an extra year of ADT?
That's my understanding of it. Your ADSO is dependent on whatever is longer, the years of sponsorship during medical school, or residency. I'm taking a 3-year scholarship, so that means I will have a 3-year ADSO, unless I am doing an active duty residency that is 5 years or greater (for some reason, I was told by my USAF recruiter that you get one "free" year of residency (internship is free?), which I don't understand completely). Anyway, if I do a 7 year residency, let's say NS, then I'll have a 6-year ADSO, as far as I understand it. However, regardless, I took the CSAB bonus, so I automatically have at least a 4-year ADSO, anyway, even though I took at 3-year scholarship...
3) I also thought, that if you do a deferred civilian residency, then you have to do additional ADT for the years in civilian residency, is this true?
No, not as far as I understand it. Since you aren't being sponsored by the scholarship (not getting paid by them) during a civilian residency, you don't accrue additional ADSO, but you probably don't accrue IRR, either.
That's my understanding of it. I may be wrong, since I'm just starting the process. Maybe some with more experience can correct my mistakes, if any.