The bank was unable to explain the cause of the problem, how many cards and machines were affected and when it is likely to be fixed.
Similar issues with ATM cards were reported late last night at its sister banks in the North and Britain. The problem meant that some ATM cards would not work on either its own machines or those operated by other banks.
For affected cards the machines themselves, and those of the other banks, told customers the service was temporarily unavailable.
A spokeswoman for the bank was contacted but did not provide clarification or a statement. The bank had earlier assured customers its ATM network and its cards would remain working despite the crisis affecting its IT system.
This had already seen 100,000 customers denied payments and unable to access their balances.
The bank said it was keeping branches open throughout the weekend and if customers required money they will have to attend in person and bring identification.
Earlier the office of the Irish Data Protection Commissioner asked Ulster Bank whether its technical breakdown was as a result of hacking.
“We are in touch with Ulster Bank to establish whether the speculation in relation to hacking is accurate,” said a spokesperson for the ODPC.
Ulster Bank has strenuously denied that its systems were subject to a malicious attack. It says the problem is a technical fault.
“We are engaging with all of our stakeholders, including the office of the Irish Data Protection Commissioner. I can assure all customers that this problem is strictly of a technical nature,” said an Ulster Bank spokesperson.
According to Ulster Bank, its parent bank Royal Bank of Scotland installed a technical patch on their systems which had unforeseen circumstances.
Meanwhile, outstanding payments to Ulster Bank customers are being processed by AIB after the bank invoked a contingency plan.
It is understood the Irish Payment Services Organisation (IPSO) was instrumental in holding crisis meetings and teleconferences to arrange for AIB to step in and help process payments on behalf of Ulster Bank.
IPSO head of communications Una Dillon said that due to the arrangements put in place yesterday, most of the 100,000 outstanding payments should be pushed through by this morning.
“We have been assured that the problem has been rectified and we are happy with what they have said, but there are no guarantees and we’ll only know on Monday morning if anything has gone wrong,” Ms Dillon said.
The IPSO is demanding a full report into what happened at Ulster Bank and is not ruling out sanctions against the bank.