Businessman Sean Quinn snr says his family lost €3.2bn in 2007 and 2008 on its Anglo Irish Bank investment.
He was a "fool" he said, and described the investment as a fiasco.
The former businessman said former chief executive officer David Drumm had accepted the Quinn family's investment in Anglo Irish Bank as security for loans during a phone call in December 2007.
Mr Quinn accepted there was no paper evidence to support that assertion.
He described as a "sham" a letter dated 31 January 2008 setting out how a number of the Quinn family's property assets were to become security for loans from Anglo Irish Bank.
Mr Quinn is the fifth witness to testify in the trial of Sean FitzPatrick, 65, from Greystones in Co Wicklow, 51-year-old Patrick Whelan of Malahide in Dublin and 63-year-old William McAteer of Rathgar in Dublin.
The men have pleaded not guilty to 16 charges of unlawfully providing financial assistance to individuals for the purpose of buying shares in Anglo Irish Bank in 2008.
Mr Whelan has also denied seven charges of being privy to the fraudulent alteration of a loan facility letter.