All ATMs at the 250 Bank of Ireland branches nationwide are out of service, including at 50 banks across the capital.
Since earlier this morning, Bank of Ireland customers have been unable to make lodgements or withdrawals.
The mobile app, 365online, and Business Online Services have also been affected by the network outage.
Customers are being advised that Bank of Ireland cards are working at other bank ATMs and in retail outlets; while credit cards can be used as normal.
Cheques and cash can still be lodged or withdrawn at branches where there is a cashier service.
It's unclear what has caused the fault on the Bank of Ireland network or how long the system is expected to be out of service.
A spokesperson has said technicians are working to fix the issue as a clear priority.