During the Troubles in Northern Ireland, there was a similarly devious counterintelligence plan against the Provisional IRA, though it didn't involve a dead body.

The context was that earlier in the conflict, a British Army Intelligence operation called the MRF (for Mobile Reaction Force) had been discovered by the Provisional IRA; in part, this was a group of plain clothes soldiers who had controversially been patrolling Northern Irish streets, occasionally taking pot-shots at people they suspected of being IRA terrorists. A less objectionable facet of their operations was that they were collecting intelligence, partly by driving IRA informants around inside Armoured Personnel Carriers identifying suspects, and partly by surveillance operations like the Four Square Laundry, where plain clothes soldiers operated an ostensibly legitimate business, using a van to pick up unclean laundry from houses in nationalist neighbourhoods and testing forensically for evidence of explosives or firearms. In 1972 the Provisional IRA uncovered an MRF double agent, found out about the MRF, ambushed the Four Square Laundry van, and held a press conference exposing what had happened. This was a considerable success story for the IRA.

Fast forward to May 1974, where two prisoners, Vincent Hetherington and Myles McGrogan, were placed on remand, accused of killing a policeman, and asked to be placed in the Provisonal IRA's wing of Crumlin Road prison. They immediately came to the attention of the IRA wing because they weren't members of the IRA, and had had nothing to do with the killing, and so they were interrogated by the other prisoners.

At first the interrogation seemed plausible - they admitted they weren't IRA volunteers, that they chose the IRA's wing because it would be the safer option, given the nature of the crime they were accused of, and that they were locked up because the RUC had beaten false confessions out of them. However, the IRA members were somewhat suspicious, and the interrogation continued over a period of days. Eventually it worked. Hetherington broke, and finally admitted that he had been approached by the RUC (Royal Ulster Constabulary - the police) over a firearms charge, and had been offered immunity from prosecution in return for becoming an informer. After this, he admitted that the police had hatched a plot for him to poison some of the more senior IRA personnel in the prison, and started naming the names of other informants inside the IRA, McGrogan included. McGrogan, for his part, continued saying nothing, though IRA prisoners found a note from him to Hetherington, threatening to murder him if he squealed. This confirmed the guilt of both Hetherington and McGrogan.

The prisoners had found another coup similar to that which had outed the MRF. They eagerly sent Hetherington's information up to the leadership, and the army started rooting out the supposed informants. IRA members on the outside (and in the other major prison, the Maze) were interrogated, with all the beatings, mistreatment and torture that implies - and even if a member survived interrogation, he was likely to resent his questioners. The whole army turned in on itself in a witch-hunt where no member could trust any other. Histories generally record this as being one of the worst periods for the Provisional IRA, (though there are other factors involved such as the 1975 ceasefire and the tit-for-tat campaign of sectarian murder).

Shortly after the shit hit the fan, McGrogan and Hetherington vanished into protective custody in odd circumstances, and were found not guilty at their trial a few weeks later.

Their story was all a lie; Hetherington and McGrogan had been planted by British Intelligence, who had coached them to give this story under interrogation in the prison. Whether that was the initial intention, or whether they were going to attempt to infiltrate the IRA first is not clear. The informants named by Hetherington were innocent, if that's the word, and the story had been carefully crafted to match the theories an average IRA member would have of how the British security apparatus worked. Dillon's book suggests that at least two or three people were killed as a result of the revelations.

The IRA eventually worked out they'd been had, and Hetherington and McGrogan were shot dead, in 1976 and 1977 respectively.

Source: The Dirty War by Martin Dillon