That's a new one to me. I've never heard of an IRS approach. Do you get a penalty if you go below minimums? Can it be audited?
Not sure what you mean. Technically, it is always an INS/IRS approach, one cannot use raw GPS data.
Depending on the RNP level of the approach, you may not be able to use the approach in the degraded mode.
The system will tell you the RAIM and availablity. Depending on the system you have different capabilities.
To answer your question, yes, with IRS drift and other issues, you will get penalties, ie reduced RNP levels or GPS N/A.
You will not get a varied minima depending on GPS avail, it is a A or NA...