INS navigates (tell it where you want to go and it will tell you how to get there - usually via the flight director and almost always can be coupled to the autopilot. The heading and attitude outputs are a useful by product of the stable platform (either virtual or physical) used to supply the navigation circuits with groundspeed and direction information) The system is stand alone.
IRS only supplies reference information (attitude, heading, speed, acceleration, etc,etc,etc. This is then fed to other systems which calculate a navigation solution (FMGS) or display the output as a compass rose or attitude ball.) The system cannot navigate, it can only provide present position information.
The details of the internal mechanisms are irrelevant. There are INS's that use RLG's and IRS's that use standard rate gyros.
Swedish,
I think you need to read up on your INS/IRS theory. All 757 and 767's have IRS fitted (there may be some private/modded ones that don't but I'm not aware of any) as do the 73 classic (i.e 300/400/500) and you DO have to put PPOS into it. How can an
inertial system tell you where you are if you don't tell it where you started!!???????
You may be confused with systems which automatically input PPOS based on GPS data. However the data still needs to be inputted (sp?)
Hope you're not flying or maintaining a/c based on this rather sketchy information??!!