Ibby writes:

The IRS system IS a system that provides positioning data of the
aircraft to the Flight Management Computer but is not as accurate as
modern GPS so my comments stand correctly. They perform the SAME
function but GPS use is more common now.

The IRS is more accurate over short distances and times, but the GPS is much
more accurate over long distances and times, whence the utility of using both.

The 747-400 uses both an IRS and GPS data. NOTAMS will display which
countries are GPS certified. The Pilot must configure the Flight
Management Computer when flying over these countries so that the the
IRS positions aren't updated with an innaccurate GPS position

GPS accuracy is not country-dependent, since it is a space-based system. I've
never heard of GPS certification for countries; can you point me to a typical
NOTAM for this?