The Taoiseach today challenged the executive director of the Centre for Public Inquiry (CPI) Frank Connolly to take legal action if he believes he has been libelled by allegations that he was involved in a plot to undermine the State.
Mr Connolly has denied being involved in a plot to train Colombian rebels in the use of explosives which involved him travelling under a false passport to the south American country in 2001.
Mr Ahern today reiterated his backing for claims by Minister for Justice Michael McDowell that "tens of millions of dollars" were to be paid in exchange for the training and that the cash was "apparently to be spent on distorting our democratic process".
There has been outcry over Mr McDowell's claims and his use of Dáil privilege to make the claims against Mr Connolly.
Last week, the CPI's funding was cut by its sole benefactor, American billionaire philanthropist Chuck Feeney.
Mr Ahern faced stiff questioning from Sinn Féin's Dáil leader Caoimhghin Ó Caoláin in the chamber this morning and was challenged to specify what group posed a threat to the State.
The Taoiseach said the issue was not Mr McDowell's actions, which have led to calls for his resignation, but Mr Connolly's abuse of the passport system. Mr McDowell leaked to a newspaper an alleged passport application baring Mr Connolly's photograph but in the name of another person.
"I didn't make up the form. I didn't place the form in the Department of Foreign Affairs. Neither did Minister McDowell. The issue is that somebody had a bogus document.
"And if somebody feels that they are aggrieved and maligned by what is said inside the House or outside the House, there is a way through the court that they can vindicate their name," Mr Ahern said.
Mr Ó Caoláin said McDowell was fantasising and that the purpose of the attack on Mr Connolly to serve party political ends.
"What exactly is the threat to the security of this state that led Minister McDowell to take the unprecedented action that he has done and with all of the potential serious consequences that it must have for the individual rights of all citizens?" the Cavan Monaghan TD asked.
Labour leader Pat Rabbitte claimed that the CPI had been planning to investigate the site for the proposed new prison in north Co Dublin before its funding was withdrawn.