Greetings,
The IRS (ADIRU) does not measure the wind 200 nm ahead of the aircraft, it is the FMS who is doing the job, by blending the wind entered in the wind page, with the wind sensed by the ADIRU at present position, then the FMS applies a percentage of the resulting blending as per the distance between the present position and the TO waypoint (the active leg) .
For the Pegasus it is 200 nm, for the Thales Smith it 100 nm ( I know, the FCOM says 200 but it wrong)