This is what we're being taught in flight school right now:
IRS / INS / IRU are essentially different names for stable platforms which provide
1.) pitch, bank and yaw information by using whatever fancy gyro / laser technology suits the budget
2.) position information by mathematically integrating data which was received from accelerometers strapped to the platform
In modern airlines there are ADIRU units which couple IRUs with an air data computer to feed the complete flight data package directly to the EFIS.
Then there's AHRS (Attitude Heading Reference System), which only provides pitch, bank and yaw information. There are no accelerometers, so no position info can be derived. Can be found in light bizjets such as the CJ1 we've just received
Also featured in the G1000 and electronic standby instruments.
Hope that's all correct so far
