In a word it’s Mountbatten.
When the maternal uncle of the Queen of England was blown to pieces along with family members and friends as they enjoyed a day’s sea fishing off the Co Mayo (that should have read Co Sligo) coast on a warm August day in 1979, Martin McGuinness was Chief of Staff of the Provisional IRA and as such would have known of the plan for some time and given it his approval.
Otherwise it wouldn’t have happened.
Assume then that somewhere along the line McGuinness had been turned and was now working for the other side and had briefed his handlers about the plot to kill Mountbatten. What would his handlers do? Intervene to save Mountbatten or let him die so as to preserve a precious intelligence asset?
I would guess they would try to arrange matters so that Mountbatten lived and McGuinness escaped blame in some way, the finger of suspicion not yet turning in his direction.
Well, only one of those outcomes actually happened. Mountbatten and his holiday party were blown to pieces. I can’t imagine British spookdom seeing anything positive about that. This story doesn’t give an answer to the ‘Was McGuinness an informer?’ chatter but it needs to be included in the conversation.
The Broken Elbow 