Property developer Mr Tom Gilmartin is to reject an alibi given by the Taoiseach to the planning tribunal over a claimed meeting involving the two men.
Mr Ahern is expected to tell the tribunal, which today starts the long-awaited hearings into Mr Gilmartin's allegations, that he was at a function in Glasnevin when the developer claims he met him and other Fianna Fáil politicians in Leinster House in February 1989.
However, Mr Gilmartin is sticking to his account of the meeting, and is expected to point to the evidence of former government minister Ms Mary O'Rourke, who says she attended a meeting involving Mr Gilmartin and senior ministers around this time.
In an opening statement to be delivered this morning, tribunal lawyers will outline the results of their five-year investigations into the allegations made by Mr Gilmartin. The developer says he was subjected to repeated demands for money as he tried to get official support for two shopping centres in the late 1980s.
At least 40 witnesses are to give evidence in this phase of the tribunal's investigation into the developments at Bachelors Walk and Quarryvale in west Dublin.
They include Mr Ahern, former Taoiseach Mr Albert Reynolds, the former EU Commissioner Mr Pádraig Flynn and five former Fianna Fáil ministers. Other witnesses will include Mr Flynn's daughter Ms Beverley Cooper-Flynn, the former Fianna Fáil TD Mr Liam Lawlor, two of Mr Ahern's closest aides and a variety of gardaí, planners, county councillors, civil servants and business figures.
The present module will cover events from 1987, when Mr Gilmartin returned from England to promote his plans for large shopping developments at the two sites, to 1990, when mounting debts and official hostility forced him out of the projects. Later this autumn, the tribunal will hold separate hearings into allegations that bribes were paid to have Quarryvale rezoned in the early 1990s, when Mr Gilmartin was no longer involved.
Mr Gilmartin met a series of high-ranking politicians in 1988 and 1989 to lobby for his projects. In June 1989, he gave Mr Flynn a cheque for £50,000, with the payee left blank. The developer says he intended the money for Fianna Fáil, but it has recently emerged that the sum made its way into a bogus offshore account held by Mr Flynn and his wife.
After the February 1989 meeting with senior ministers, Mr Gilmartin says he was subjected to an extortion demand. A man approached him in the corridors of Leinster House and told him to deposit £5 million in a bank account in the Isle of Man.
Mr Gilmartin refused and says his building projects went awry after this date.
Most of the politicians who Mr Gilmartin says met him in Leinster House deny they were in attendance or, as in the case of Mr Ahern, say they can't remember.
It is unclear whether Mr Charles Haughey, who Mr Gilmartin says was present, will be called to give evidence.
The former Taoiseach's name does not appear on the list of witnesses.
Tribunal lawyer Mr John Gallagher is expected to lead the evidence in this module.
After his opening statement, Mr Gilmartin will be the first witness, with the others to follow next week.
Although the hearings are likely to be highly embarrassing for Fianna Fáil, the fact that many of the witness statements have been leaked in recent weeks is expected to reduce the shock value of Mr Gilmartin's allegations.
