The bank has not yet said what is causing the difficulty for many users who are trying, and failing, to log in
Bank Of Ireland suffered an online banking outage at lunchtime on Thursday.
Attempts to log in to its online banking apps and website were met by error messages.
Dozens of customers have taken to social media to complain about the outage.
“We are aware that customers are experiencing an issue logging into mobile banking,” the bank said in a statement.
“We are working to fix this as quickly as possible and apologise for any inconvenience caused.”