I might suggest that many of the posts above whilst "technically correct" from an FCOM point of view, really have an incomplete idea of how an Airbus works.
I've a few thousand hrs driving them, and would never pretend to really understand what goes on under the hood
The interactions between systems are far more complex than our FCOMs lead us to believe, as numerous incidents have shown (some in our airline)... where investigations show FCOMs as incorrect, and even Airbus taking sometime to work out what/how happened (I have one Flight Control query in with them via my airline and am told we might hear their response early next year!).
This incident seems a very rare (isolated?) one, and suggest we leave the ATSB and Airbus to figure it out. Exactly where an ADIRU 1 feeds into the the FCCs, in various laws/modes, and with which switch selections will be interesting
NoD