I'm pretty sure this has been done to death before on Pprune (try 'search'?) but my understanding (as FE H's) is that
IRS = Inertial REFERENCE system ie pitch/bank/yaw (used as part of an..........
INS = Inertial NAVIGATION system ie where are you etc.
Both INS and IRS can be based on EITHER Solid State or mechanical gyro platforms.