A loyalist paramilitary claim MI5 asked them to assassinate the Irish prime minister in 1985 is among revelations in newly-released Irish state papers.
Documents include a letter purportedly written by the Ulster Volunteer Force (UVF) to Charles Haughey, saying an MI5 officer asked them to "execute" him.
The documents also contain a claim Sinn Féin leader Gerry Adams "set up" an IRA gang who were shot dead by the SAS at Loughgall, County Armagh, in 1987.
Sinn Féin describe that as "nonsense".
The claim was passed on to the Irish Department of Foreign Affairs by the respected Catholic cleric, Fr Denis Faul, about three months after the Loughgall killings.
The details are all contained in previously classified documents, held by the Irish National Archives, which have been made available to the public for the first time under the 30-year rule.
The 'UVF' letter, which was sent to the taoiseach (Irish prime minister) in 1987, contained a long list of collusion allegations against the British intelligence services.
Addressed to Mr Haughey, it read: "In 1985 we were approached by a MI5 officer...he asked us to execute you."
It stated that UVF paramilitaries had been supplied with details that would have compromised the taoiseach's personal security, including aerial photographs of his family homes, his cars and his private yacht.
The late taoiseach's son, Seán Haughey has confirmed to Irish broadcaster, RTÉ, that his family were aware of the death threat at the time and took it seriously.
The disputed claim about Mr Adams followed a foiled IRA bomb attack on police station in Loughall in May 1987.
Eight IRA men were shot dead by undercover soldiers as they loaded a 200lb bomb onto a stolen digger and smashed through the gates of the barracks.
The late Monsignor Denis Faul - a former Maze prison chaplain who was credited with helping to end the IRA hunger strike in 1981 - told Irish diplomats later that year that there was a theory that "the IRA team were set up by Gerry Adams himself".
The then Fr Faul had attended school in County Tyrone with one of the IRA men who was shot dead in the ambush.
He told diplomats he was "intrigued" by the theory, which has since been dismissed by Sinn Féin.
"These claims are utter nonsense," a party spokesman said in response to the declassified file.
Months earlier, the Irish government was told that another influential Catholic cleric believed Mr Adams was planning a future IRA ceasefire in 1987.
The view was expressed by the then Bishop Cahal Daly, who later became the most senior Catholic cleric on the island of Ireland.
His remarks were contained in a confidential report, compiled for Irish government officials.
A diplomat reported: "The bishop has picked up a rumour that Gerry Adams is currently trying to put together a set of proposals which would enable the Provisional IRA to call a halt to their paramilitary campaign.
"He has reached the view that the "armed struggle" is getting nowhere, that it has become a political liability to Sinn Féin both north and south and that, as long as it continues there is little chance that he will be able to realise his own political ambitions."
'Loss of face'
The report, dated 4 February 1987, continued: "What he is believed to be working on is some form of 'declaration of intent' to withdraw, with however long a timescale, on the part of the British government.
"If he managed to negotiate something of this kind, the Provisional IRA would be able to lay down their arms without much loss of face, claiming that they had achieved the breakthrough towards which all their efforts had been directed."
The document also revealed Bishop Daly had expressed his distrust of the Sinn Féin leader and spent time "agonising" over whether or not to meet him.
Mr Adams had a better personal relationship with the west Belfast Redemptorist priest, Fr Alec Reid, and it has previously been reported that the pair were discussing a possible end to IRA violence as early as 1982.