The PWS is triggered from a microswitch in the Throttle Stand. It shares the same switch as the T/O Config Warning trigger. In other words, when the throttles are approx vertical, independent of WXR switch position.
Next time you get in the cockpit with IRS turned off, advance the throttles until the T/O warning sounds. You will also see a 'PWS FAIL' on the ND.
Without investigating, I've always thought the IRS had to be aligned, so the a/c could identify the windshear in a relative position - but could be wrong.
Point to note; while taxiing onto the ramp, if you use so much power that the T/O warning sounds (however unlikely), you're frying the ground staff with PWS, even if WXR turned off...