The bank has apologised after many people were unable to use their bank cards of make online transactions between 9-11pm on Wednesday night.
It is the second major problem for the bank in less than a year, after thousands of people were unable to withdraw cash or use their accounts for weeks over the summer.
"This problem was caused by a hardware fault and was not related to the issues we experienced last summer," said a spokesperson.
"It was much easier to fix, though clearly an unacceptable failure.
"We apologise for the disruption our customers experienced last night. All systems are up and running as normal."
In a statement, the bank urged customers with problems to get in touch and hinted at compensation.
Lauren Goodwin is just one of the bank's Northern Ireland customers left without access to their cash.
The student first realised there was a problem when she tried to return items to a clothes shop.
"It was quite embarrassing because there was a long queue behind me," she told UTV.
"When I checked my card and realised it was not my fault, it was relief for me."
Online banking was also down and as Lauren could not withdraw cash from ATMs, she said she did not know what was happening with her account.
"It's really frustrating as a whole and I hope something can be done about it," she said.
It was another letdown, another chance that Ulster Bank had not allowed people to get their money out.
The disruption is understood to be part of a UK wide problem which parent company RBS said has since been resolved.
"We are disappointed that our customers have faced disruption to banking services for a period this evening, and apologise for that," the bank tweeted.
Following a major computer fault which caused disruption during the summer, Ulster Bank paid out more than £18m to almost 300,000 customers in Northern Ireland.
Last month the bank reported losses of more than £1bn for 2012, but that does not include the payout given to affected customers.
The bank plans to close seven branches and four sub-offices in NI by this June.
The Consumer Council called for assurances that the issues have all been fixed.
"The Consumer Council asked Ulster Bank three specific questions: what had caused the problem; if consumers could be assured it wouldn't happen again; and whether customers' accounts and balances were up to date," said Antoinette McKeown.
"We have received assurances that the fault has been identified and resolved and we are continuing to work with Ulster Bank in relation to the questions we have posed."