Inruder is not fully right. It is not possible that the gyro is just keeping the north during fllight.
On ground it is determined from the turn of earth, that's why we have to provide the system accurate initial position and the airplane must be steady during alignment.
Laser gyro could be very accure, but it is impossible that it is not collecting any error in several hours.
If we put an IRS into "ATTITUDE" mode than the heading must be entered manually. This heading has to be updated manually in every 10-15 minutes, like on a mechanical gyro. But the gyro is the same than in normal IRS operation.....