IMU vs IRU
First post after finding this thread about 150 or so pages ago, so be gentle.
From everything I've read here and other sources, the IR part of the ADIRUs only rely on gyros -- and do not include accelerometers -- whereas IMUs use both. Also from what I've read, it appears that for the ADIRU the gyros are contained within the IR part of the box in the avionics bay, not externally (and there really is no reason for them to be).
So, as this has not yet been confirmed as part of AF 447 I have to go with HazelNuts' suggestion and for now assume it might be a part of an Ariane (there just happened to be a launch that included two boosters on July 1st).