Don't know about all IRS's, but the one in the 767's still use basically the same accelerometers as the old style INS's do.
That is, an arm that hangs out and is held in place with an electric motor, and there's a couple of light sensors that detect movement, which then feed back to a box that sends power to the electric motor to keep the arm in place. The amount of current used is related to the G-force being acted upon the arm.