Gerry Adams and Martin McGuinness held lengthy negotiations with the former Irish prime minister Bertie Ahern to save the Northern Ireland peace process in the full knowledge that the IRA was planning to carry out the biggest bank robbery in its history, according to leaked US cables passed to WikiLeaks.

Ahern, who was instrumental in drawing up the 1998 Good Friday agreement, judged that the two Sinn Féin leaders were aware of plans for the £26.5m Northern Bank robbery in 2004 because they were members of the "IRA military command" with a deep knowledge of its operations.

The US cables also reveal that:

• The Irish government believed Britain had a "valuable source of information" at a senior level in the republican movement.

• Adams argued that the IRA would have to be "taken out of the equation" during negotiations which led the organisation to declare a formal end to its armed campaign in July 2005.

The revelations are published as Adams seeks to broaden Sinn Féin's appeal in the Irish Republic. The Sinn Féin president is abandoning his Westminster seat to stand in the forthcoming general election amid hopes of a breakthrough as voters register anger with Ireland's mainstream political parties after the country was forced to apply to the EU and IMF for a bailout.

Ahern's concerns about Sinn Féin and the IRA are highlighted in cables which describe a challenging period in the peace process as London and Dublin sought to restore the power-sharing executive in Northern Ireland. Unionist suspicions about the intentions of the republican movement were fuelled when the IRA robbed the headquarters of Northern Bank in Belfast in December 2004.

In a cable on 4 February 2005, two months after the robbery, the US ambassador to Dublin, James Kenny, reported that a senior Irish government official told the embassy of the taoiseach's concerns about Adams and McGuinness. The cable claimed the official in the department of justice told the ambassador "that the GOI [government of Ireland] does have 'rock solid evidence' that Gerry Adams and Martin McGuinness are members of the IRA military command and for that reason, the taoiseach is certain they would have known in advance of the robbery".

In another cable on 1 June 2005, six months after the robbery, Kenny reported that Ahern had raised his concerns with Mitchell Reiss, the US envoy to Ireland. The cable says: "The taoiseach … believes Sinn Féin leaders were aware of plans to rob the Northern Bank even as they negotiated with him last fall. Publicly, he has been unprecedentedly critical of Sinn Féin and, until recently, greatly reduced private contacts as well."

The cables indicate that in private Ahern and officials used language which was slightly blunter, though consistent, with the public pronouncements of the former taoiseach, who told the Irish parliament, the Dáil, he believed Sinn Féin had negotiated in bad faith. Ahern told the Dáil on 2 February of a meeting with police chiefs on both sides of the Irish border. "They believe that a number of operations which took place during 2004, not just the Northern Bank robbery, were the work of the Provisional IRA and would have had the sanction of the army council and be known to the political leadership."

Sir Hugh Orde, the former chief constable of the Police Service of Northern Ireland who met Ahern, accused the IRA of carrying out the robbery. In what was then the largest cash robbery ever carried out in the UK, a group of armed men held the families of two bank officials hostage while the officials were forced to hand over sacks filled with millions of pounds in cash to terrorists at the bank.

Nobody has been convicted of any offence in relation to the actual robbery. Ted Cunningham, a Cork-based financial adviser, was found guilty last year of laundering more than £3m connected with the robbery.Adams has consistently denied being a member of the IRA. McGuinness has admitted being a member in the 1970s.

A Sinn Féin spokesperson said: "There is not a shred of evidence that has ever linked the IRA to the Northern Bank robbery. The theories put forward by the British at the time regarding republican involvement were disproved in court. No link has ever been made, other than by opponents of Sinn Féin, that the IRA was involved. All the republicans arrested in connection with the Northern Bank robbery were released without charge."

The cables also suggest:

• Adams was a powerful voice in arguing that the IRA had to stand down during negotiations in the run-up to the Provisionals' historic statement in July 2005 of a "formal end to the armed campaign". In the cable on 1 June 2005, the US ambassador to Dublin quoted Adams as saying: "The IRA must be taken out of the equation."

• Michael McDowell, the former Irish deputy prime minister, said after the murder in 2006 of Denis Donaldson, a British informant working for Sinn Féin, that he believed Britain had a more senior mole. In a cable on 31 May 2006, a US diplomat wrote: "McDowell believed that the outing of Denis Donaldson as an informant was a clear message from the British government that it had another, more valuable, source of information within the republican leadership. He reiterated the taoiseach's point, however, that Sinn Féin leaders appeared to have had no connection to Donaldson's murder."

• An intriguing Anglo-Irish role reversal in which Dublin, normally regarded as the guardian of republican interests in the peace process, felt that Tony Blair had gone too "soft" on Sinn Féin. The cable sent by the US ambassador on 1 June 2006 says: "GOI concerns about UK 'softness' represent a role reversal. Usually, it is the UK that is concerned Ireland will be too accommodating to Sinn Fein."

• Brian Cowen, the current taoiseach, claimed in 2005 that Adams had played a "double game on criminality". In a cable dated 8 March 2005, the US ambassador to Dublin James C Kenny wrote: "Cowen [then finance minister] ... believed that, after the May Westminster elections, Sinn Féin would attempt to convince people of its seriousness about criminality through actions designed to back up the party's recent positive rhetoric on the subject. Cowen related his impression that Gerry Adams was playing a 'double game' -- taking a hard public line against criminality, but avoiding definitive action in order to retain maneuverability for final negotiations with unionists."

A Sinn Féin spokesperson said: "Brian Cowen is a political opponent of Sinn Féin. As a former finance minister and now as taoiseach he has brought the country to its knees."