Mr Charles Haughey said he refused to approach people whom he speculated may have contributed up to £1.6 million for which the tribunal could not find a source.
The tribunal was told that from January 1988 to October 1992 it appeared £3.3 million was available in funds for the benefit of Mr Haughey but the sources of £1.6 million was unknown.
Mr Haughey said he did not know who they were and would not speculate.
Mr John Coughlan SC, for the tribunal, asked: "Can you be of any assistance to the tribunal on the sources of what appears to be in excess of £1.5 million?"
"This tribunal has all the powers, it uses them comprehensively and if you can't come across any other evidence of contributions made to me or to someone on my behalf, I can't help you," Mr Haughey said.
He said most ordinary people these days would be in "total horror" of the idea of being brought before a tribunal.
Mr Coughlan said that if Mr Haughey suggested names, investigations would be carried out privately. Mr Haughey said that would be of no consolation to an individual who would be terrified.
"It's both that I don't know who they might have been and, secondly, I'd have to refuse to speculate."
Mr Coughlan said: "Well, if I might take this to its final logical conclusion: would you be prepared yourself to approach people whom you might speculate about and ask them would they be prepared to assist you?"
When Mr Haughey asked in what way, his lawyer Mr Eoin McGonigal SC said: "To go out in the highways and byways, Mr Haughey, and see if you could find somebody who might have contributed to the £1.6 million that Mr Coughlan has been unable to find a source for, that's the question."
Mr Coughlan said that was the suggestion. Mr Haughey replied: "No, I wouldn't be prepared to do that, Mr Coughlan, if you ask me a specific question about a specific person but otherwise I wouldn't. I don't think I should be asked to do that."