Ulster Bank has said 78 of its branches will remain open until 7pm today, and open on Sunday if necessary, to assist customers affected by a delay in processing certain payments.
The bank has been hit by serious technical problems resulting in a two-day delay in processing some payments to individual bank accounts.
More than 100,000 customers have been affected, with Social Welfare payments, salary transfers and direct debits disrupted.
Ulster Bank said concerned customers should check with their branch or the website for updates.
Pensioners whose pension payments have been delayed by the computer problems will be able to withdraw funds by calling to their nearest branch, Age Action has been informed.
The bank says the issue has now been fixed but it will take the weekend to clear the backlog, which amounts to millions of euro in transactions.
Minister for Social Protection Joan Burton called on Ulster Bank management to "get on top of their game".
She said it was a matter of absolute urgency, particularly for older people and those relying on Social Welfare payments.
Ms Burton said she was extremely disappointed that the bank had not acted with greater speed to address the problems people were facing.
The problem has also affected customers of other banks whose employers process their wages through Ulster Bank.
HSE employees who are due wages can withdraw up to €500 from their own banks on production of a payslip and an ID card, according to arrangements between Ulster Bank and the HSE.
Around 40,000 HSE workers are affected by the problem.
Ulster Bank's Chief Operating Officer Ellvena Graham said the bank apologised unreservedly for the "massive inconvenience" caused to its customers.
She said it had taken longer than anticipated to resolve the technical problem, which was discovered on Wednesday morning.
The bank has said there is a 48-hour backlog in sorting out payments and bank balances.
Ms Graham said the error originated in the larger group IT system within Royal Bank of Scotland - Ulster Bank's parent bank - and had to be dealt with by the group's IT staff.
She said a new software ''patch'' that was being applied to the group system had caused the problem and that it had now been removed.
Ms Graham said staff would be working over the weekend, and she anticipated that everything would be ''back up and running'' properly by Monday morning.
She also said that customers of other banks, who had been affected by the Ulster Bank problem and who had incurred charges, would be reimbursed.
Ulster Bank says 50 branches in the Republic and 28 branches in NI will remain open until 7pm for customers who are unable to get to their branch during working hours.
Branches that normally open on Saturday will have their hours extended to 3pm, and if necessary, some will open on Sunday.
