ULSTER Bank customers were told to seek compensation from the bank after its second payments collapse in months.
The bank insisted its systems were now back working after thousands of customers were denied access to ATMs, could not make online transactions, and could not use debit cards.
This is the second time in the past nine months that customers have been locked out of their funds due to technical errors.
Central Bank regulators told customers to seek compensation from the bank if they had lost out.
"Impacted customers should make a claim if they have not already done so to Ulster Bank," a spokeswoman for the regulator said.
The Consumers' Association of Ireland said the latest technical problems at the bank raised serious concerns for customers. CAI chief executive Dermott Jewell said the bank was not delivering an adequate service.
Mr Jewell said: "It affected quite a significant number of people who needed to access the system to download money, to take care of their banking needs and they couldn't. And realistically, it needs to be borne in mind that they are paying for this service and it's just not up to scratch."
He said the lessons from the IT systems collapse at Ulster Bank last June had not been learned. The bank was paralysed for almost a month last summer.
"If we hark back to June, the lessons from then do not seem to have been made clear to the customers of Ulster Bank to explain what has happened, what has been put in place to make sure there's a system that's robust."
Ulster Bank said it was sorry. "We apologise for the disruption our customers experienced. All systems are up and running as normal, though any customers with any individual problems should get in touch with us."
For three hours on Wednesday night, customers were not able to access online banking and had problems with ATM use and point-of-sale payments.
The bank insisted that the problem was caused by a hardware fault and was not related to the systems collapse last summer.
"It was much easier to fix, though clearly an unacceptable failure. Any customer who was left out of pocket due to this outage should get in touch so we can put things right for them."
Last month, the bank was also forced to apologise when it emerged that it had failed to collect the correct mortgage repayments from 1,300 customers. €40m has been underpaid.
Last November, the bank was fined €1.96m by the Central Bank after it breached rules on how much day-to-day cash it holds to cover deposits and rainy-day capital for potential losses.