The liquidators of IBRC, formerly Anglo Irish Bank, have admitted the lender was engaged in overcharging for a 14-year period up to 2005.
The disclosure was made by the special liquidator for the bank, Kieran Wallace of KPMG.
He was testifying in a case in the Bankruptcy Court in Delaware where the liquidators are defending a challenge to the winding up the bank in the US.
In a statement, Mr Wallace said the majority of variable rate loan customers in the Republic of Ireland, Isle of Man and the US had been overcharged between 1990 and 2004.
Approximately 25% of loans in the UK were overcharged for 14 years up to 2005.
Mr Wallace said the issue had been identified in June 2010.
There had been a forensic investigation which led to refunds for customers and compensation.
The spokesperson for the liquidator said: "Mr Wallace informed the Court that allegations of overcharging of interest to customer loan accounts for subsequent periods have been examined by IBRC internally prior to the special liquidation of the bank, and by the Special Liquidators since their appointment on 7th February 2013.
"The special liquidators are satisfied that there is no evidence of deliberate, systematic overcharging of interest to customer loan accounts in subsequent periods (ie post 2004/2005)."
The court heard that the liquidation of IBRC, formerly Anglo Irish Bank, has cost €5m since it began in February.
US Billionaire Paul Singer and Irish developer John Flynn are taking the action against IBRC.
Mr Singer has been granted limited disclosure of documents regarding the operation of liquidation of the bank.
Mr Singer is questioning the legality of the liquidation.
Mr Flynn is taking an action in which he is claiming he was overcharged by IBRC.
Despite Mr Wallace's assertion that the issue of overcharging had been dealt with, an expert working for Mr Flynn disputed that this was the case.
Eddie Fitzpatrick of consultancy Bankcheck said clients of IBRC had been over-charged by a total of $1.6bn.
Mr Fitzpatrick said the problems still persisted.
In a statement this evening, the spokesperson for the liquidator said two matters were contested before the US court today.
The first concerned an application for wide ranging discovery taken by the Burlington 1 Hedge Fund in their objection to the Chapter 15 application being taken by the Special Liquidators.
This has been refused.
The second concerned the application by Mr Flynn and others to lift the provisional stay, "which is in place pending the IBRC bankruptcy hearing to allow the Flynns to proceed with the legal action brought by them in New York."
The judge refused the application and the "stay will remain in place pending the outcome of the recognition hearing under Chapter 15 of the US Bankruptcy Code."