Two questions here.

The first is, was it the IRA who walked into Castlereigh and took the files. Anything's possible, but it doesn't seem too likely on the face of it. On the other hand, it would be very convenient for some people if the blame could be paid there, for example because that would take the heat of the other suspects inside the British intelligence structure.

The other question is whether there is basic information about British politicians in IRA files. I think that is about as likely as the suggestion that there might be information about IRA activists in official British files. It'd be extraordinary in either case if there weren't.

In any case neither of these things amount to a breach of the ceasefire.