I will back up Spanner's answer with confidence on ALL ISIS systems. ADIRU/IRU's etc supply heading info only, yes the ISIS will operate with all other functions W/O IRU's aligned. The only question I forgot the answer to was that the ADIRU provides a reference to North, I believe if you align the responsible IRU and ISIS is up then the IRU is lost heading will still be operational on the ISIS as it had allready been given true north info.