Property developer Mr Tom Gilmartin said he was subjected to death threats as a result of his involvement with the tribunal, writes Paul Cullen.
He said he, his son and a friend in Cork received at least four telephoned death threats between 1998 and 2003.
He told the tribunal that in the first, which he received at his home in Luton, he was told in strong language to "remember Veronica Guerin". If he did, he would know what was waiting for him if he turned up in Dublin to give evidence to the tribunal.
He said that on another occasion, his son was told by a caller that if he came to Dublin to give evidence, he wouldn't be going back. Another caller threatened to "make a good job of me". A friend in Crosshaven, Co Cork, had received another threat.
Mr Gilmartin said he believed the calls were made from Ireland. The callers had Irish accents, one from Dublin and another with a hint of a Cork accent.
He said he didn't tell the English police because they would have connected it to "an IRA thing". In addition, the stress on his wife, who has MS, would have been "disastrous".
Mr Gilmartin also revealed that men claiming to represent the Provisional IRA tried unsuccessfully to extort money from him when he was living in England. Three men barged into his house in Luton one evening demanding money for Noraid, he said.
Mr Gilmartin said he told them that "not a penny" of his money would end up buying bullets to kill anyone. He wouldn't consider the death of anybody.
"They told me I'd better cough up, but they said they would be back on the Thursday. I said: 'You'd better make a good job of me because I'll take two of you with me'." The men probably didn't belong to the IRA because most of those who "mouthed off" about membership of that organisation weren't in it, he concluded.
Mr Gilmartin said he informed then minister Mr Pádraig Flynn of this episode, probably in the context of telling him about the demand for money he was facing at that time in Dublin.
Separately from these allegations, he claimed to the tribunal that an unidentified man threatened to throw him into the Liffey in 1989 after he refused a £5 million extortion demand.
Earlier, Mr Gilmartin claimed that the former assistant Dublin city and county manager, George Redmond, interfered with his plans to develop a large shopping centre at Quarryvale in west Dublin. He said Redmond was "in the pocket" of rival developer Green Property Group, and alleged that the official's staff were afraid to speak out against their boss for fear of losing their jobs. He accused Redmond of "acting the clown" at a meeting between council officials and Mr Gilmartin and his team in February 1989. That morning's meeting was cancelled, resulting in an "altercation" between the two men, the witness said.
Mr Gilmartin said he was outraged at what had happened, because he had been made look like a fool. He complained to Mr Seán Haughey, who was the deputy city manager and the brother of then Taoiseach, Mr Charles Haughey.
The meeting was reconvened in the afternoon but before it started, Mr Haughey and Redmond had a heated discussion.
According to Mr Gilmartin, Mr Haughey asked his colleague: "What's your game?"
"Ask your brother," Redmond is alleged to have replied.
"I'm not my brother's keeper," Mr Haughey is said to have responded. "Your games are going to stop, I can assure you."
Mr Gilmartin said Redmond spent the meeting making wisecracks and "acting the clown" when he was trying to discuss serious issues.
Mr Gilmartin's account is supported by that of one of his associates, Mr Richard Foreman, who has told the tribunal that Redmond was "deliberately obstructive" during the meeting.
Mr Foreman said the meeting started in an odd fashion with an altercation between Mr Haughey and Redmond. His statement describes Redmond's attitude as "at best offhand, at worst rude".
In 1989, Mr Gilmartin reached agreement with the chief valuer, Mr Michael McLoone, to buy corporation land at Quarryvale for £40,000 an acre. However, he claims Redmond tipped off Green Property about the sale. Green bid for the land and Mr Gilmartin was forced to pay significantly more for the land when it was put up for public tender.