no_moolah, this is even more serious. The financial institution has proactively provided false information to the ICB. There is no outstanding balance. The bankruptcy order had the effect of writing the balance down to zero, unless it was a joint mortgage and the other party wasn't made bankrupt. The Data Protection Commissioner should be serving the financial institution with an enforcement order. My tuppenceworth would be to approach it in this order.
1) Start with a letter to Helen Dixon, the Data Protection Commissioner, outlining the situation. This is a new area and she is probably unfamiliar with the scenarios that are developing.
2) If no response, engage with your local TDs. Attend their clinics, and ask them to intervene with Helen Dixon on your behalf.
3) If still no movement, consider civil legal action against the financial institution.
4) And to put a bit of pressure on, formally advise the financial institution's auditors that incorrect information about your account balances appears to be maintained on their ledgers.
This is also a problem in the UK. The below post on Money Saving Expert in the UK outlines how the problem is resolved there. Some of the template letters referred to may be of use, but they would need to be edited to refer to the Irish equivalent. Also, Helen Dixon might be interested in the process her UK counterpart, the Information Commissioner's Office, uses to resolve these issues.
http://forums.moneysavingexpert.com/showthread.html?p=7834457#post7834457