How about:
INS = older, self contained nav system with mechanical gyros and accelerometers.
IRS = an integrated inertial reference system using RLGs with no navigation function but outputs to navigation and other systems (FMS, autopilot, wx radar instruments etc.)
...and a period of transition where aircraft like the TriStar and 747 were built with old INS systems integrated into the design and fulfilling the functions of a modern IRS.