The Central Bank has said that it is "closely monitoring" the situation at Ulster Bank, as tens of thousands of the bank's customers struggle to access their cash.
It had been thought that around 100,000 customers were affected but it is now understood that the bank is now working on a two-day backlog, rather than one.
The problem is affecting any payments made from the 19th of this month, but the bank said mortgage payments will have been debited even if customers' account histories indicate otherwise.
"Ulster Bank can confirm that we continue to experience a major IT issue which is resulting in a delay in processing certain payments and we apologise unreservedly for this," the bank said in a statement on its website.
"We will be keeping approximately 80 Ulster Bank branches open until 7pm in major towns and cities to assist customers who are unable to get to their branch during working hours and our 60 Saturday opening branches will open as usual this weekend," the statement added.
"If you are trying to access a salary or benefit payment and you cannot access your funds via an ATM, you may wish to visit a branch.
"If visiting a branch, please bring your ID and payments details."
The bank said that customers who experienced any charges or loss of interest would be refunded fully.
Jobs and Enterprise Minister Richard Bruton today urged the bank to speedily resolve the situation.
"They have to do everything in their power to get this right, and also to protect customers," he said.
"I know there are an awful lot of customers who are worried if this will have a knock-on effect on their credit rating."
Among those affected are HSE employees, 40,000 of whom found their accounts were not showing any wages payments.
The HSE’s national director of finance, Liam Woods, sent an email to all staff to tell them that there would be a delay in salary payments.
A spokesperson for the Department of Social Welfare said 29,000 social welfare customers have also been affected by the glitch.
The problems have also affected a “small number” of customers of Ulster Bank parent Royal Bank of Scotland (RBS) in the UK, as well as customers of sister bank NatWest.
Head of card services at the Irish Payment Services Organisation Una Dillon this morning said more people are now expected to be caught up in the problem
"Many people would be paid on a weekly or fortnightly basis, either on a Thursday or Friday," she said.
"There would be a lot more people expecting to be paid today that won't have those funds over the weekend."
The issues with Ulster Bank technical systems have had a knock-on effect on customers in other banking institutions as well. It is understood that no payments (debits or credits) were received from Ulster Bank. This has affected customers in other banks such as AIB and Bank of Ireland who were expecting payments that have not been processed by Ulster Bank.
Meanwhile on Twitter, dissatisfied customer expressed their frustration at the situation:
HSE payroll is one of biggest in the country, this #ulsterbank fiasco is hurting an awful lot of families who don't have any surplus income— Ken Curtin (@kencurtin) June 22, 2012
2nd day now and still not paid!..great banking system #ulsterbank— ♏️AnGELo⚽️ (@emer4612) June 22, 2012
Although one commenter offered a moment of levity:
Dear #UlsterBank. Have you tried turning it off and turning it on again— Annie West (@anniewestdotcom) June 21, 2012
Additional reporting: Irish Examiner