Sorry; Inertial Reference System; IRS. Nowadays each one uses laser ring gyros rather than mechanical ones, to detect rotations in each axis which can be integrated, (I think), to give velocity and distance travelled in each axis from their starting point.
On Airbus FBW there are three independant units, which are further enhanced with air data inputs and are called ADIRS, three of which are fitted to each Airbus FBW, and combined with two GPS receivers to give very comprehensive position data.
So five systems - not just one system - for position, and even then flying a safety margin above MSA.