PRNAV1,
I was waiting for one of the technocrats to come up with something edifying here, but in there absence will give a rough explanation (Now, watch the technocrats attack this one like bees to a honey pot).
Laser Ring Gyros, the primary component of the IRS, detect movement by measurement of the frequency shift of a laser emission as the aircraft moves. Light moves at a fixed speed, it is not subject to the vector sum of it's own speed and that of the originating source, that is, if light moves at 186000 miles per second, and is transmitted by an aircraft moving at 500 knots, it still moves at 186000 miles per second, NOT 186000 miles per second plus 500 knots. Observation of the laser transmission will therefore be obvious to a moving object (the aircraft) as a frequency shift from the constant speed light source (much like Doppler). This can be measured, and converted mathematically to relative speed.
Laser Ring Gyros typically fire laser pulses over a triangular course (contained within the aircraft of course), thus, longitudinal, vertical, and lateral components of movement can be measured. When this movement is referenced to the original position, the current horizontal and vertical position, speed, etc. are known.
Various spectrums of 'Normal' light, and other radio frequencies, as we know them, do not move at exactly the same speed, there is a slight variation in speed between frequencies, thus the astronomical observation of 'red shift' of light from receding galaxies (Red light moves faster). For an effective IRS, the speed must be very stable and predictable, and this is offered by the very pure and frequency stable laser emission.
There are no gyros, tilting mechanical tables etc. contained within an IRS, it has no moving parts, that description belongs to INS. Laser gyros are not gyros per se, but the equivelant thereof.
That's my two bob's worth, now, flak jacket on, awaiting incoming from the technocrats.
Regards,
Old Smokey