Here's a link to an online version of Chesteron's Man Who Was Thursday Very well worth reading, even if you have to do it online. Greg summarises that aspect of it well.

The suggestion I've seen is that Alfredo Scappaticci was fingered by "elements within" the British Intelligence in the hope and expectation that the IRA could be induced to execute him and so be accused of breaking the peace in a public way. This would leave open whether or not he has been a British agent. In fact that wouldn't make much difference either way. There is nothing so readily disposed of as an agent who is past his sell by date.

Of course, no doubt at some time it'll turn out that British Itelligence had been infiltrated by the IRA. This thing gets murkier and murkier.