The IRS is a standalone system that cannot be corrupted by GPS spoofing or jamming (unless of course the GPS position which you used to initialise the IRS was somehow corrupted, but I assume that is not what you are talking about).
If you do have some form of GPS outage your ANP will start going up. In the cruise this is unlikely to be a major issue as you still have IRS and Radio Position available. Not sure why your are asking about RNP. RNP is dictated by what and where you are flying, not by how good your GPS signal is.
I am sure all this is covered in your FCOM and FCTM