Freddie Scappaticci was the infamous double agent (Stakeknife) who headed the internal security unit for the IRA.
http://www.newsletter.co.uk/news/stakeknife-probe-will-look-at-ira-role-1-7699108
http://www.belfastlive.co.uk/news/stakeknife-linked-murder-probe-uncovers-12025740