A US court has been told that the former Anglo Irish Bank overcharged clients by more than €500 million.
The allegation was made by Eddie Fitzpatrick, who runs a forensic banking firm called Bankcheck.
Mr Fitzpatick's claim is part of litigation being taken by Irish developer John Flynn, who is making allegations of fraud against IBRC, which was formerly Anglo Irish Bank.
Mr Flynn is objecting to the liquidation of IBRC because it could block him being paid damages if he won his fraud case.
IBRC is fighting attempts to frustrate the wind-down of IBRC, which is now subject of a series of objections in the US.
It has said it would vigorously oppose attempts to block the liquidation of the bank.
In new documents lodged in US Bankruptcy Court in Delaware, Mr Fitzpatrick said he had audited 720 separate customer accounts.
He found the clients had been overcharged as a result of "loading" of interest rates ranging from 0.5% to 0.05%.
Mr Fitzpatrick said: "All of those accounts were subjected to both manual and automated deceitful procedures which resulted in substantial overcharging of interest to their accounts totalling in excess of €7.7m.
"In my opinion the extent of the loading of rates and the lack of correlation in the amount by which the rates were loaded by would lead me to conclude that the conduct of the bank could have been nothing other than deliberate or to put it another way an attempt to defraud customers."
However, he said in order to verify the claim he need further records from the bank.
Following the release of the Anglo Tapes, Mr Fitzpatrick requested transcripts of recordings relating to interest rate loading.
The special liquidators of IBRC have applied for bankruptcy protection to prevent €1bn of assets being seized by creditors while the bank is being wound down.
Mr Flynn is among a series of objectors in the US to the liquidation.
The others include US billionaire Paul Singer, whose companies owns $75m of IBRC subordinated loans.