The fundamental difference between an INS and an IRS is that:
The INS is a strap down system with 2 accelerometers mounted 90º to each other
The IRS is a strap down system with 2 accelerometers mounted 90º to each other
The INS is a strap down system with 3 accelerometers mounted 90º to each other
The IRS is a strap down system with 3 accelerometers mounted 90º to each other