An INS is a self contained navigation system, i.e. you can program waypoints and the system will direct you point to point. It generates its own attitude platform and inertial position information (like an IRS). It also provides attitude reference. Think of an INS as an IRS with a navigational computer integrated into one system.
An IRS is a inertial REFERENCE system only. It provides attitude reference and present position information (along with some misc info like speed, heading etc). The position information is provided to separate flight management systems (FMS) that do the navigational computations with pilot interface.
INS systems are rapidly falling from use due to the increased adaptation and cost effectiveness of Flight Management Systems that use IRS laser-ring gyro systems for basic position/attitude inputs.
INS systems like the Carosel and Sperry are very expensive to maintain due to the complex mechanical components and are now outmoded. Properly maintained they are reliable and provide good accuracy, but cost and reliablilty becomes a major factor.
[ 15 August 2001: Message edited by: Roadtrip ]