The property developer, Tom Gilmartin, has accused the Taoiseach of lying over a meeting he claims he had with Mr Ahern and other members of the cabinet in Leinster House in February 1989.
Mr Ahern's barrister had told the Mahon Tribunal that the Taoiseach will say that he has no recollection of attending any such meeting.
Mr Gilmartin said recollection seemed to be a nice word for getting around the house rules. He said Mr Ahern was actually lying.
At the proceedings today, Mr Gilmartin also retracted earlier evidence that he told Ms Harney about the meeting.
He had earlier claimed that he told Ms Harney about the meeting when he attended a house warming in Dalkey. He now accepts that this party took place in December 1988, two months before he claims the Leinster House meeting took place.
Probe hears claims about Flynn
Earlier, the tribunal heard claims from a British property executive that former minister Pádraig Flynn asked him for 'a donation to the boys'.
Ted Dadley of Arlington Securities said that when he refused, his company's plans for a development at Bachelor's Walk in Dublin were exposed in The Irish Times causing acquisition costs to soar.
In a statement made to the tribunal, Mr Dadley said Mr Flynn made the request after lunch in a London hotel. He claimed the Taoiseach Bertie Ahern also attended the meal.
Meanwhile, during cross-examination, developer Tom Gilmartin denied that it was his idea to hire Liam Lawlor as a consultant for a property deal at Bachelor's Walk.
The tribunal was told Mr Dadley would say they agreed to pay Mr Lawlor £3,500 a month for 'political PR prowess' at Mr Gilmartin's suggestion.
Mr Gilmartin has told the tribunal that Arlington decided to pay Mr Lawlor against his wishes, after the TD gatecrashed a meeting of executives in London.
But a statement from another executive, Raymond Mould, said Mr Lawlor was never at such a meeting.