A Times journalist, who clearly is suspicious of the Martin McGuinness as 007 thesis that has recently emerged, explores the ramifications of that particular idea:
http://www.timesonline.co.uk/article/0, ... 25,00.html
Here's the crux of his point:
In the Troubles, where almost everything on the surface was a lie or a deception, the only hard certainties are the bullets, the bombs and the dead bodies left in the ditch. In the summer of 1984 Patrick Magee, a veteran bomber, acting on the orders of the IRA’s Army Council, really did plant a bomb behind the bath panel of room 621 of the Grand Hotel. When that bomb exploded six weeks later it very, very nearly killed Margaret Thatcher and most of her Cabinet.
If McGuinness had been an MI6 agent then MI6 must have known about the Brighton bomb and then allowed the potential decapitation of the British Government to go ahead. Therefore MI6 was really behind the Brighton bomb. Instead of being plausible the thesis that McGuinness was a MI6 agent becomes nonsensical.