Bank of Ireland has been fined €24.5 million by the Central Bank.
The fine relates to the company's failure to have a proper contingency system in place in the event of a serious IT disruption to its services.
The Central Bank says deficiencies were repeatedly identified from 2008, but it took 11 years for the bank to fully address them.
Bank of Ireland was initially fined €35 million, but that was reduced by 30% under a settlement discount scheme.
The Central Bank says BoI admitted to five contraventions, which occurred between 2008 and 2019.
Seána Cunningham, the regulator's director of enforcement, said modern banks are "wholly dependent" on reliable and resilient IT systems.
She said: “Without an effective IT service continuity framework, significant IT disruptions, particularly if they were to happen in a bank, could have a very serious impact on millions of customers who rely on ready access to their funds and services to keep their everyday lives and businesses moving.
“From 2008 until 2019, BOI was in breach of key regulatory provisions regarding IT service continuity, arising from deficiencies that were repeatedly identified between 2008 and 2015 in third party reports. However, steps to address these deficiencies only commenced in 2015.
“The extent and duration of these breaches were particularly serious given the ‘always on’ nature of the services BOI provides and how pivotal IT is to the entirety of its business operations."
She said the impact of the breaches meant that BOI "may not have been able to ensure continuity of critical services" had a severe disruption happened.
In a statement, Bank of Ireland said they've "co-operated fully, proactively and voluntarily" during the investigation.
They said: "Bank of Ireland fully acknowledges, and sincerely apologises for, each of these breaches which should not have arisen.
"To comprehensively address these breaches the Bank has invested heavily in IT service continuity, completing an extensive Groupwide programme of work between 2015 and 2019.
"This has included technology investment such as infrastructure and network upgrades, and enhanced testing, planning and internal procedures."
