The original how long is a piece of string question. Broadly, and very generally :
Specifically INS rather than IRS.
If the Aircraft was fitted with INS from new then the answer is – Probably yes. The possibility exists that rather than a third INS for Standby purposes it might be a tray of Gyros instead and just be an Attitude/Heading source. I’ve seen both.
If the INS was retro-fitted then the answer would be probably no, it’s normally too much work to rewire all the equipment that need Attitude and Heading ( think about it ).
This is a VERY general answer, I now expect a plethora of responses “We retro fitted our fleet of Mk IV Scrungejets in 1975 and replaced all gyros” etc.