The jury heard details of the two men's backgrounds
Two former IRA prisoners blackmailed two businessmen to get a share of £6m, Southwark Crown Court has heard.
Nick Mullen, 60, from Acton, west London, was once alleged to have been a "quartermaster" at an IRA bomb factory.
His co-accused Ronald McCartney, 55, from Belfast, was convicted of trying to kill three policeman in the 1970s.
The London court was told that they demanded the men pay £150,000 each "or face the consequences". Both deny two counts of conspiracy to blackmail.
A third man, Louis O'Hara, 43, from Loughton, Essex, also denies two counts of conspiracy to blackmail between 1 January and 16 April 2008.
All three are accused of "conspiring together and with others to make, with a view to gain for themselves or another, unwarranted demands for payment of monies in the sum of £150,000 with menaces".
Nom de guerre
Mark Heyward, prosecuting, told the court that the defendants claimed the victims had raised £6m using the name of the IRA and they felt the men "would have to contribute in the sums demanded or face the consequences".
"In a nutshell, the prosecution case is that these three defendants, with others, entered into an unlawful agreement to extort money from each of the two victims by making these threatening and menacing calls, letters and visits," he said.
"It is difficult to imagine a more concentrated kind of threat than the threat purported to have come from a paramilitary organisation, like the Irish Republican Army, notwithstanding the lessening of the Troubles and also the Good Friday Agreement in the late '90s," he said.
Mr Heyward also said that letters to the alleged victims - who cannot be named for legal reasons - bore the Irish Gaelic for the IRA - Oglaigh na h-Eireann - and were signed with the organisation's nom de guerre "P O'Neill".
"You will hear that is an historical signature often used by the Provisional IRA in the course of the height of the Troubles to sign off official statements," he said.
The court was told that the first threatening call was made on 26 March last year.
A man claiming to be from the IRA and using the name P O'Neill allegedly told one of the businessmen: "There has been an investigation. We know what you've been up to."
Mr Heyward said the call came from Mr McCartney's mobile phone and was "akin to downright extortion, the words pregnant with the threat of violence".
Later, in a second call, the barrister said the caller warned: "You better make delivery."
The barrister also read out a letter allegedly sent to the businessmen which said: "When you think of the volunteers who are in early graves as a result of [the Republican] struggle and the volunteers and families who have endured years of imprisonment you will understand our justifiable anger that the two of you have exploited this situation for your own personal gain."
The letter added: "If you involve the police or anyone else then Fort Knox will not be safe for either you or your extended family.
"We want to resolve this issue, but if you fail to comply then appropriate action will be taken."
Prosecutors say the letters were "intended to instil sufficient reaction, sufficient fear" to force them to hand over the money.
The court was told that Mr McCartney "accepted" he had known one of the alleged victims for more than 10 years, but "denied any involvement in sending any blackmail letter or writing it".
Mr Mullen, meanwhile, also admitted knowing one of the businessmen and writing one of the letters, which included the words "their patience was limited".
Ronald McCartney was convicted of attempted murder in 1976
He insisted he was simply "asking for a donation".
"He stated he was just a middle man, an honest broker," said Mr Heyward.
Opening what was expected to be a three-week trial, the barrister told the jury that the nature of the case meant it was appropriate for them to know aspects of the defendants' backgrounds.
Mr McCartney, the barrister said, was convicted at Winchester Crown Court in 1976 of "three counts of attempted murder of three different policemen".
He said he was also found guilty of "a single count of conspiracy to cause explosions, possessing a firearm with intent to endanger life and using a firearm with intent to resist arrest... all of which related, says the prosecution, to his activities as a part of his membership of the IRA".
Mr Mullen, meanwhile, was convicted of "significant terrorist offences and was sentenced at the Old Bailey to 30 years' imprisonment in June 1990 for conspiracy to cause explosions".
"He was alleged in that trial to have acted as quartermaster for an IRA active service unit in London in 1988," he said.
"However, and you will also hear evidence of this, in 1999 the Court of Appeal Criminal Division... quashed his conviction on the principal ground that the process used to return him to the United Kingdom from Zimbabwe where he had gone after the events that had led to his arrest, was itself unlawful."
The trial continues.