Ulster Bank has apologised to customers after a technical fault meant that many of them failed to receive automated salary payments and social welfare benefits today.
The fault is understand to have also affected transactions at Natwest, its sister bank in Britain.
The bank, which is Ireland's third largest, says it is attempting to fix the problems as soon as possible.
Staff at Ulster Bank headquarters and at branches across the country have spent much of the day dealing with complaints and queries from customers.
A technical problem overnight left many people without scheduled payments into their bank accounts.
The bank's website was also affected, with internet customers informed that account balances were not up to date and online services were unavailable.
Ulster Bank would not put anyone forward for interview today, but said that its automated and online systems had been affected and that the bank was working hard to have the problem fixed as soon as possible.
It is also understood that Natwest, which is also in the Royal Bank of Scotland group, experienced similar problems earlier today, but that they have now been rectified.
People expecting social welfare payments were also affected and the Department of Social Protection said it was aware of the problem.
