It is suspected by some that the UK government was able to turn one or two high-ranking IRA members into confidential informants.
It is suspected by others that this is only a deliberately planted rumor for UK psychological operations.
I recall something coming out a few years ago. Just did a search and found this about “stake knife”, which is what I was partly recalling.
http://www.cbsnews.com/2100-202_162-553436.html
There are a few other articles
http://www.guardian.co.uk/politics/2003/may/18/uk.northernireland1