IRR can usually be ignored, as personnel in the IRR aren't typically activated/mobilized to active duty. But, it IS possible; just not likely.
As far as commitment time for HPSP (i.e. scholarship + IRR = 8 years) --> If you're a 4 year HPSP scholarship recipient who does a military internship, 4 years of AD payback, you will still have 3 years of IRR when you separate from the service. Usually you'll be in residency for those 3 years, and again, it's highly unlikely that you would be mobilized, but it is possible.
Essentially, if you haven't served 8 years of AD when you separate from service (as stipulated in your HPSP contract), then that remaining time must be served as an affiliated reserve medical officer (IRR, MIT, drilling reserves, etc.).