Don't get confused between Inertial Navigation Systems and Inertial Reference Systems. An IRS only knows where it is, an INS not only knows where it is but also where it should be. Any difference between the two is an error -- drift. The INS is working the desired track between two waypoints from the flight plan. If wind blows the aircraft off course, the INS detects the drift and can compute windspeed and direction from this. Most Inertial Navigation Systems can be coupled to the Autopilot/Flight Dirtector and any tendency to drift is automatically corrected. Just add autotuning to an autocoupled INS and Bingo! - you get an FMS.
**************************
Through difficulties to the cinema