How does the attitude and heading reference (AHRS) work on Boeing 737 aircraft? I would like to know how the AHRS work on Boeing 737 aircraft. If this aircraft (like every the other ones I guess) has an Inertial Reference System (IRS), composed of 3 accelerometers and 3 gyroscopes, what elements compose its AHRS? 
3 accelerometers and 3 gyroscopes as well?
The 737 does not have a separate AHRS. Those functions are simply outputs from the two IRS's.

(Source) 737 IRS panel.

ATT (attitude) mode selects the AHRS-only output of the IRS.