The Sinn Fein finance spokesman said: "Customers deserve a full explanation of why their money has disappeared from their accounts".
In a statement, Ulster Bank said it is 'aware some transactions previously applied to customers accounts since April 20 are no longer showing.
"Card has been declined checked the app and shows my balances not as they should be", another customer said.
The bank has told impacted customers, who find their accounts overdrawn as a result of the problems, that they will not be out of pocket once the issue is resolved.
"We investigated this issue as a matter of urgency and have already taken the necessary corrective action which will see recovery actions overnight".
Customers are being asked to not resend payments. They are asking affected customers to call 1850 424 365 (from overseas 00353 1 8047475) minicom - 0800 015 4422. Need a brand new podcast?
The bank notes that not all customers are affected by this.
Some Ulster Bank customers have claimed that money has gone missing from their accounts.
One customer said: "Standing orders/direct debits due to hit".
The Irish Times is reporting that the issue is not expected to be resolved until Monday evening. Extraordinary how often this kind of issue happens with this bank.
Another stated: "My salary has disappeared from my account today and my wife had her card declined when trying to pay for a GP visit and medication".
The Joint Committee on Finance and Public Expenditure agreed this afternoon to invite Ulster Bank to Leinster House.
They are offering those customers emergency cash of 500 euros (£437).
A massive technical meltdown at Ulster Bank in 2012 left huge numbers of its customers without access to funds for weeks and led to the Central Bank imposing a fine on the bank of €5million for the IT failure. "Time to change to a competitor I think".
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