Ulster Bank has apologised after customers using its ATMs were charged twice for withdrawals earlier this week.
The problem affected customers taking money from cash machines in Northern Ireland between 4.15pm on Easter Monday and 4.15pm on Tuesday afternoon.
"We are working to correct this for affected customers and restore money to accounts overnight," a spokesperson said.
"Customers will not be left out of pocket as a result and can contact us if they require further assistance. We apologise for any inconvenience caused."
The bank was unable to provide further details on the nature of the system failure, but said that the issue had now been resolved.
Ulster Bank, part of Royal Bank of Scotland, was one of the worst affected by a major outage in 2012 which prevented access to accounts for millions of customers across the UK.
In January, a senior member of the Prudential Regulation Authority, Sam Woods, told a Northern Ireland Affairs Committee meeting that the outage had highlighted the "antiquated" IT systems used by banks in the UK.
Woods said: "Despite our progress, I feel that we are a very long way from being able to sit here with confidence and say UK and Northern Ireland systems are robust."