Does anyone know or understand the process for an IIR Soldier to be medically boarded?

ADOS orders are for a specified mission. If you are starting a MEB, then more than likely you would not be able to fullfill the duties and responsibilities of the ADOS requirement, due to all the appointments associated with the board. You should be on medical orders for the MEB either in lue of your ADOS or after completion (keeping you on active status through your board).

The process would be no different than for any Soldier from the other reserve components/status (i.e., USAR/ARNG TPU, IMA/DIMA, etc). As with all reserve component Servicemembers there will have to be a Line Of Duty (LOD) to show that the injury was sustained while on active military service (not "Active Duty", active military service ... i.e., "on the job").

The process is covered in AR 600-8-4*, para 5-4 (Entitlement to medical care, pay and allowances for Soldiers who incur an injury, illness, disease, or incur aggravation in the line of duty).
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* AR 600-8-4 - https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN33106-AR_600-8-4-001-WEB-2.pdf

I've personally never heard of or experienced meeting someone on IRR being medboarded. I went through the process myself and never saw somebody from IRR. Good Luck.