As Wolfgang pointed out "Some subgroups reminiscent of the former glorious action days and not feeling fit for the political way of Sinn Fein, could use their military training and knowledge of weapons for a more private purpose.

This seems a far more likely explanation than one involving a deliberate action by the IRA in consultation with the Sinn Fein leadership.

If there was any political motive behind the robbery, this could only have been intended to stymie the peace process, and damage Sinn Fein. That could still be consistent with it having been carried out by Republicans opposed to the process.

Always remember, any speculation here is done from outside the loop, with only partial information, which is drip-fed to us through the media.