The Supreme Court has rejected a bid by Cork property developer Owen O'Callaghan to stop the Mahon tribunal inquiring into allegations made against him by Tom Gilmartin.
In a 4/1 majority verdict, the court found Mr O'Callaghan had failed to show the tribunal was objectively biased against him or had prejudged his credibility unfavourably.
In a dissenting judgment, Mr Justice Hardiman found the tribunal had failed to treat Mr O'Callaghan fairly to date, that it has prejudged certain vital issues and that Mr O'Callaghan and associated companies are unlikely to be treated fairly in the future.
The High Court rejected Mr O'Callaghan's case last October, but he appealed it to the Supreme Court.
The date for the Taoiseach's appearance at the Mahon tribunal, to respond to allegations made by developer Tom Gilmartin about payments in the early 1990s, was put back following hearings in Mr O'Callaghan's Supreme Court case.
The so-called Quarryvale Two module of the tribunal, in which the Taoiseach will be one of the star witnesses, was due to begin on January 16th.
In an opening statement at the start of its investigations into Quarryvale last year, the tribunal said Mr Gilmartin had claimed Mr O'Callaghan had informed him that nearly £80,000 had been paid to Mr Ahern.
In the statement read out prior to the court challenge, the tribunal quoted Mr Gilmartin as saying he was told that the money had been paid in two separate amounts, of £50,000 and £30,000.
Mr Gilmartin said he was informed that Mr Ahern, who was minister for finance at the time, had been instrumental in blocking a tax break for a rival shopping centre being developed in Blanchardstown.
Mr Ahern and Mr O'Callaghan have strongly denied this allegation. Mr Ahern reminded the tribunal that he had successfully taken a libel action in relation to earlier claims that he had received money on behalf of Mr O'Callaghan.
In its opening statement last year, the tribunal also maintained there were allegations that corrupt payments had been made to 14 Dublin county councillors. The lobbyist Frank Dunlop has said he paid £232,000 to councillors.