Ulster Bank has been fined a total of €4.6m by the Central Bank for governance failings relating to targets the regulator set in 2013 to tackle deep mortgage arrears across the banking sector.
Ulster Bank has admitted the breaches, which took place from 2013 until 2015.
The Central Bank said its investigation found that Ulster Bank fell "far short" of the governance standards applicable to the preparation of regulatory returns.
It also found serious failings in Ulster Bank's approach to the compilation and submission of its Arrears Resolution Targets returns.
Today's fine is the first levied by the Central Bank for such breaches.
The Central Bank had implemented measures to resolve the high volume of mortgage arrears that developed in the wake of the financial crises and introduced the Mortgage Arrears Resolution Targets (MART) Framework in 2013.
Banks were required under the MART Framework to report details on the level of mortgage arrears on their books to the Central Bank on a regular basis.
The Central Bank said it had informed Ulster Bank of its governance failings around the compilation of its MART returns in 2013, and the bank said it would take action.
But it was not until 2015 that Ulster Bank actually acted to address the issues.
"This inaction and delay by the firm is unacceptable and the Central Bank is treating such failure by the firm as an aggravating factor in this case," the Central Bank said.
It also said a further aggravating factor was Ulster Bank's previous enforcement record, which had repeatedly identified poor governance practices at the lender, which is owned by Royal Bank of Scotland.
The Central Bank had determined that the appropriate fine for Ulster Bank was €6.572m, but it was reduced by 30% to €4.6m in accordance with the settlement discount scheme provided by it.
Seána Cunningham, the Central Bank's Director of Enforcement and Anti-Money Laundering, said accurate and reliable data is vital to the Central Bank's ability to monitor risk and supervise firms effectively, and to ensure the ongoing stability of the financial system.
"Effective governance is a fundamental part of ensuring data quality. Firms must satisfy themselves that they have robust, effective and tested governance in place to ensure the quality of the data they manage and disseminate. If we cannot trust the data supplied by firms, we cannot do our job," she added.