The ICB, which is owned by financial institutions including AIB, said it was first alerted to the problem six weeks ago by AIB.
Chief Executive Seamus O Tighearnaigh said the ICB and AIB have been working together to deal with the problem.
He said the Data Protection Commissioner was informed and letters were sent to customers affected last weekend.
He said what happened was wrong and the ICB would now write to its other members to alert them.
The Office of the Data Protection Commissioner said it was a serious breach of data protection law; at the upper end of the scale.
Deputy Commissioner Gary Davis told RTÉ News that AIB had supplied inaccurate personal data to the ICB.
As a result, he said, people who applied for credit may have been turned down, with serious consequences.
He said such people may not even know why their application was not approved.
He said the commission’s initial focus has been on getting AIB to remedy the problem and this had turned out to be a far more lengthy process than envisaged.
The commission conveyed its disappointment to AIB at the delay in resolving the problems.
He said now the problems had been remedied, the commission has moved on to try to understand how this serious issue happened.
He said the commission is more broadly concerned that the issue may not be isolated to AIB and so is beginning a generalised audit of financial institutions.
He urged every affected customer to get a copy of their credit report to check if it was now accurate.
And he said where such people have had credit applications turned down in the past six years they may have grounds to resubmit their application.
The Central Bank said it is aware of the issue.
If impacted customers have queries, they should contact AIB at the helpline number provided in the letters to customers and make a complaint if necessary.