"- .1/FLR/FR0906010211 34123406IR2 1,EFCS1X,IR1,IR3,,,,ADIRU2 (1FP2),HARD"
As I read the section on analyzing ACARS messages in the BEA preliminary report, I read it that "identifiers" indicate additional units that submitted messages during the 60 second fault window. These messages will have had the leading 3/4 (or was it 2/3?) digits of the message the same as the message reported. Therefore we know(?) that IR1 and IR3 also reported something that began with 34, or 341, or 3412, depending on how many leading digits were matched.
The above "analysis" is based on my memory of the pages in the BEA report which I read a couple weeks ago, so the details are fuzzy. It is also based on the assumption the BEA description is accurate; and that I interpreted it accurately. I'd encourage others to reread the BEA description, or even better, some original description in a maint. manual if you have one.