Dream - my understanding is that that system is not certified for sole-use navigation UNLESS updated by DME/DME etc. Yes it will tell you where it THINKS it is, which may not be where it really is (and I have seen 30 mile errors in NDB only land) but it will always give you attitude 'REFERENCE', hence the 'R'. Most SOPs accordingly call for basic navigation checks on receipt of the 'IRS only' warning.