The tribunal chair Judge Peter Smithwick revealed that three former IRA members were dumped by investigators after they refused to aid the review into the murders of two senior policemen in 1989.
The Smithwick Tribunal is investigating allegations of Garda collusion in the IRA murders of senior RUC officers Harry Breen and Bob Buchanan, just north of the border in Co Armagh.
The Provisional IRA members, one of whom is believed to have direct knowledge of the killings, had answered some questions in private but refused a further meeting with tribunal lawyers or to be questioned at the inquiry.
Instead, an unsigned statement from the IRA members was read into the record at the tribunal on Friday.
They said the RUC officers had been under surveillance for almost a year before they were killed.
They insisted the murder was carried out by a unit from South Armagh and rejected claims that another unit in south Down or a British agent, Peter Keeley, knew about the plot.
They also denied claims of collusion from anyone in Dundalk Garda station, including a former detective sergeant Owen Corrigan.
Mr Kenny criticised their refusal not to answer further questions, saying those involved should "now have the decency to give up the information with is necessary for the Smithwick tribunal to do its work".
Whilst the IRA members have answered some questions in private that cannot equate to a proper process of scrutiny within a hearing of the tribunal.
Jeffrey Donaldson, DUP MP
His criticism comes as Sinn Féin leader Gerry Adams renewed calls for a truth and reconciliation forum to be established.
Reacting, Mr Kenny said his calls were something that "could be applied to those Provisional IRA members".
However the party president said the IRA member's cooperation was "historically unparalleled".
"Clearly this would not have been possible but for the tribunal creating the context to allow it. I commend this," Mr Adams said.
Sinn Féin deputy leader Mary Lou McDonald said the evidence given by the former IRA members was "very comprehensive".
Later, several former Ulster Unionist members denied claims they had any knowledge that a senior Catholic RUC officer was a "likely cause of collusion" in the case.
Lady Sylvia Hermon, widow of late RUC Chief Constable John Hermon, former First Minister David Trimble and Jeffrey Donaldson denied the claim which was made in a 2002 NIO document.
Mr Donaldson, now DUP Lagan Valley MP, criticised republicans for not cooperating, saying they should give evidence to the tribunal to back up their calls for the truth about the past.
"Republicans are regularly calling for a truth commission to be established, including most recently Gerry Adams," he said.
"However, whenever there has been an opportunity for republicans to demonstrate any evidence that they are prepared to tell the truth it has always been ducked.
"Whether it was Martin McGuinness hiding behind a so-called 'code of honour' whilst giving evidence to the Saville Tribunal or IRA members now refusing to give oral evidence to Smithwick it represents an unwillingness to actually examine the past in an even-handed and transparent manner."
He added: "The families of Chief Superintendent Harry Breen and Superintendent Buchanan deserve to know the full truth about what happened to their loved ones."