Bank of Ireland has been fined €24.5 million by the Central Bank.
The regulator says the bank failed to put proper systems in place to protect its services from significant IT outages for 11 years.
It was issued to Bank of Ireland on Tuesday but its details have just been released.
The fine relates to the bank's failure to have a proper contingency system in place for customers, in the event of a serious IT disruption to its services.
The Central Bank says deficiencies were repeatedly identified from 2008 onwards but steps to address them only began in 201, and it was 2019 before they were complete.
The bank has admitted five breaches in total.
It was initially ordered to pay €35 million but that was reduced by 30 percent under a settlement discount scheme.