Hmm, extrapolating from my 738 knowledge here - the IRS itself is not a 'means of navigation' by itself in my opinion, but an FMC using information from a number of IRS is. Thats why single FMC installations don't qualify for navigation outside the range of conventional navaids, but twin FMC setups do... After all you can't enter the number of IRS in your FMC flight plan but you do have to state if you have one or two FMCs (or even more)
