Bank of Ireland said it is working to fix an issue with its mobile app and Banking 365 online banking platform.
Customers have been difficulties accessing the online services since this morning.
"We are extremely sorry that customers are continuing to experience difficultly in accessing Banking 365 and mobile banking services," the bank said in a statement.
"We are continuing to work on this as a top priority."
"We have increased the number of colleagues available to serve customers over the phone and will also open more than 90 of our branches tomorrow, Saturday, from 10am to 2pm," Bank of Ireland said.
It added that all other services are operating normally including ATMs, debit and credit cards, as well as Business Online.
Customers have been taking to social media throughout the day to complain about the problem.
The first messages started to appear on Twitter shortly after 9am with users describing difficulties logging on to online banking and the mobile app.
