Ulster Bank has been fined almost £2.75 million (3.5 million euro) after an IT failure prevented thousands of customers being able to use their accounts over a month.
The fine is the highest imposed by the Central Bank of Ireland and has been accepted by RBS-owned Ulster Bank.
About 600,000 customers were unable to access cash at bank machines and make payments, and many saw delays in getting money into their accounts during 28 days in June and July 2012.
The meltdown was described by the Central Bank as "an unprecedented disruption".
The fine was handed down after the Central Bank found Ulster Bank failed to put in place "robust governance arrangements" to prevent and deal with the IT failure.
It said the technological breakdown not only caused "widespread and significant loss" to the bank's customers, but also threatened confidence in the banking retail sector.
A software upgrade was blamed for the disruption.
The Central Bank's director of enforcement, Derville Rowland, said: " As the provision of financial services to customers represents the core business function of the firm, the major breakdown in the firm's provision of these services as a result of IT failings is completely unacceptable."
Ms Rowland said the size of the fine indicated the scale of the disruption.
"It reflects the seriousness with which the Central Bank views the failings of the firm and the Central Bank's determination to ensure that customers have access to core banking services without disruption," she said.
To date Ulster Bank has paid out more than £46 million (59 million euro) to customers as part of a redress scheme ordered by the Central Bank.