Lawyers for the Taoiseach have accused Mr Tom Gilmartin of having a "selective memory" about the events under investigation by the tribunal.
Mr Conor Maguire SC, for Mr Ahern, said there were numerous inconsistencies between the accounts Mr Gilmartin gave of his allegations at different times.
Mr Gilmartin denied that his memory was selective, but acknowledged that he had made minor errors and "slip-ups" in his evidence to the tribunal and other forums.
Mr Maguire, in his cross-examination, examined a number of matters in which he said the witness had given inconsistent answers. He said Mr Gilmartin had mixed up dates and amounts of money, transposed his projects at Quarryvale and Bachelors Walk and given conflicting accounts about the identity of the man who allegedly sought £5 million from him in Leinster House.
He pointed out that a senior garda and the city manager, Mr Frank Feely, had separately recorded that Mr Gilmartin told them that Mr Liam Lawlor was the man who demanded £5 million in February 1989.
He said Chief Supt Hugh Sreenan had noted this during a phone conversation with Mr Gilmartin in March 1989.
Mr Gilmartin said this was not correct. At no time had he said that Mr Lawlor was the person who demanded this money from him. Chief Supt Sreenan must have obtained his information "elsewhere". There was some "mix-up" and he could not account for it.
But Mr Maguire pointed out that Mr Feely had also recorded Mr Lawlor as the person who sought the £5 million, again following a conversation with Mr Gilmartin. The deputy city manager, Mr Seán Haughey, had attended this meeting and initialled Mr Feely's notes.
Mr Gilmartin said Mr Feely was confused. There was definitely a mix-up. Chief Supt Sreenan may have got his information from Mr Feely, he suggested. The meeting with Mr Feely and Mr Haughey had lasted 3½ hours, he pointed out, and covered many topics. He had not seen Mr Feely take any notes.
Mr Hugh O'Neill SC, for Mr Gilmartin, pointed out that although Mr Maguire had described Chief Supt Sreenan's notes as contemporaneous, they were dated December 2003.
Mr Maguire said the witness had made a "slip-up" in his evidence regarding details of an agreement he had made with developer Mr Owen O'Callaghan.
Mr Gilmartin said this was a "minor error". The document had been altered after he signed it.
