Well, in a nutshell, an AHRS provides you with attitude / heading information while the IRS also gives you position information (and with that also data like GS, true track, Wind data, magnetic heading / track)
If you take a look at current jet aircraft, the IR / IRU systems have a ATT position. When switched to ATT, the IRS is basically just an AHRS system (but you'll have to provide a heading reference manually)
All solid state, laser ring gyros stuff in these days.
Couple an AHRS equipped aircraft with GPS and you basically have the same information available as an IRS equipped aircraft.
Last edited by Ka8 Flyer; 19th Nov 2012 at 17:01.