In a gyro stabilised platform, alignment with True North will occur when the East gyro cannot sense any component of earth rate along its sensitive axis. Any component of earth rate causes an apparent wander of the East gyro. The output of this wander causes the platform to tilt about East resulting in the North accelerometer detecting a component of gravity. The accelerometer output is used to torque the azimuth gyro and the platform until the output from the East gyro is zero. The platform is then aligned. The accelerometer output is also fed to the levelling gyros and motors to return the platform to the level.
This principle still applies to an IRS with a Ring Laser Gyro, but the platform doesn't physically move - the corrections are digitally computed and applied.
Within the value of earth rate affecting the E/W accelerometer is a component dependent on the cos.lat. Therefore, for an aircraft at very high latitudes, this component gets very close to zero and makes accurate alignment to True North virtually impossible. However, there are INS around now with GPS integration modules that allow in-flight alignment, and I believe that these work well at all latitudes, provided the GPS is also working well!