The Royal Bank of Scotland's Irish unit Ulster Bank has been hit with a £2.75m fine for an IT meltdown in 2012 which left customers unable to use their accounts for a month.
The Central Bank of Ireland revealed 600,000 customers were affected when an Ulster Bank computer failed in the overnight processing of account information, leaving people unable to receive funds, pay for goods or access their accounts in any way during the period.
The central bank's director of enforcement, Derville Rowland, said the IT meltdown resulted in a "major breakdown" which had caused "unprecedented disruption".
The central bank added that when RBS took over Ulster Bank's IT systems in 2005, there was a failure to properly understand the infrastructure or the risks associated with outsourcing.
"Her remarks on the pivotal role played by information technology in modern banking are welcome - as are her comments on the need for the banking system to have robust IT governance arrangements to ensure continuity of service," said Larry Broderick, the general secretary of the Irish Bank Officials' Association (IBOA), in response to Rowland's comments.
"The Ulster Bank incident highlights that there are major risks inherent in devolving control of core technological operations beyond the immediate control of the institution.
"If being part of the same banking group is not enough to guarantee continuity of service, then surely relationships with third-party service providers must be even less certain."
Ulster Bank has so far paid £45m (€57m, $72m) in compensation to affected customers.
RBS and its subsidiaries are likely to face further action from Britain's Financial Conduct Authority (FCA) after a series of other IT outages over the last two years.
In the last quarter of 2013, RBS revealed that it would compensate customers who were affected by the mass outage to its online and phone banking services, which potentially left millions of people unable to purchase goods or receive payments, echoing the debacle in 2012.
Only a couple of days later, RBS's Natwest banking systems were hit again but this time by a distributed denial of service attack (DDoS), which deliberately flooded its networks with disruptive traffic.
In August 2012, RBS revealed that it had put aside £125m to compensate thousands of customers affected by a two-week computer breakdown, which led to millions of customers being stranded and unable to pay for outgoing bills.
In April this year, Britain's financial watchdog said it will investigate RBS over its 2012 computer failure that left millions of customers unable to access their accounts.
RBS has since pledged to invest £1bn into its IT infrastructure.