Ulster Bank says an issue with money disappearing from some customers' accounts was caused by human error.
Customers have been reporting problems with their bank balance since last Friday.
In some cases money had disappeared, others were reporting their cards had been declined.
In an update, the bank says there is no connection to an IT system issue in 2012 and this problem was caused by human error after a payment file was not processed.
It also hopes to have the issue rectified overnight.
In the meantime, the bank says no customer will be left out of pocket and emergency cash of up to €500 can be accessed in branch or over the phone.
Finance Minister Paschal Donohoe says the situation must be frustrating for people.
"Look, it is a real worry for anybody. To be pretty sure how much money is your bank account - check your phone, use your ATM, use your debit card - and find out the money is not there.
"It can really worry people. Cause a lot of panic to people. It is really important now the Ulster Bank give the assurance which they have given. They are saying that nobody will be out of pocket."
TDs have called on Ulster Bank to explain why money has disappeared from some bank accounts.
The bank has confirmed some money lodged after Friday isn't showing up in accounts and some customers are incorrectly overdrawn.
Ulster Bank say emergency cash is available to customers and they are trying to fix the issue.
Sinn Féin TD Pearse Doherty says it is not the first time Ulster Bank customers have had issues.
"We now hear stories of individuals getting text messages from Ulster Bank to inform them that they're overdrawn despite the fact that it was the bank themselves that have disappeared their own customers own money.
"This comes on the back of a series of events that have happened in recent weeks in terms of IT glitches within the bank. "
Fianna Fáil's spokesperson for finance, Michael McGrath, feels the Central Bank must conduct a comprehensive review of the IT systems used by retail banks in Ireland.
"It is very distressing for Ulster Bank customers to discover that money has gone missing from their bank account or that they may have had debit card transactions declined," said Mr McGrath.
"In some cases, customers may well have missed vital payments as a result of the bank’s failure.
"Ulster Bank now needs to put its customers first. The bank’s immediate focus must be on fixing this problem and ensure that affected customers will not have their credit rating adversely affected in any way.
"These IT failures are happening all too often at Ulster Bank and the question has to be asked if it is the result of a lack of investment by the bank. It is every bank’s responsibility to ensure that their IT systems are fit for purpose."
Mr McGrath added: "The Central Bank needs to assess the state of the IT systems in retail banks operating here and ensure that banks prioritise investment in this area."
Ulster Bank customers who are incorrectly overdrawn are being advised that emergency cash is available.
It emerged today that transactions made since last Friday disappeared from some customers' accounts. Some people have reported that their cards have been declined, while other have said that lodgments are not showing up.
The bank said that emergency cash of up to €500 is available via branches and telephone, adding that not all customers are affected by the issue.
It also insisted that no customers will be left out of pocket.
No customer will be left out of pocket and should any external expenses be incurred these can be looked at on an individual basis LH— Ulster Bank Help (@UlsterBank_Help) April 24, 2018
Anyone with issues is being asked to contact the bank's customer service team online or on 1850-424-365.
Ulster Bank has said this morning that it is aware that money is not showing in some customers' accounts.
Some of the bank's customers have reported that their salaries have disappeared from their accounts, and that direct debits have not been paid.
Other customers say that their cards are being declined and their balances are not correct.
Hi Loretto, we are aware of some issues with transactions. We are working hard to fix this. If you need access to funds please give us a call or visit a branch. Sorry and thanks for your patience. NG— Ulster Bank Help (@UlsterBank_Help) April 24, 2018
Morning Padraig, we are aware of some issues with transactions. we are working hard to fix this. If you need access to funds please give us a call or visit a branch. Sorry and thanks for your patience. Thanks, DC— Ulster Bank Help (@UlsterBank_Help) April 24, 2018
I am afraid we cannot confirm as of yet if direct debits will be paid or not at this precise moment. CJ— Ulster Bank Help (@UlsterBank_Help) April 24, 2018
Ulster Bank says it is working hard to fix the issue "as a matter of urgency".
"We are aware that some transactions previously applied to accounts for ROI customers since 20/04 are no longer showing," a statement read.
"We are working hard to fix this issue and apologise for any inconvenience."