Ulster Bank made an operating loss of £310m (€382m) in the first quarter of the year, driven by bad debts and amid what it called " “exceedingly difficult” market conditions in Ireland.
The figure was however an improvement of almost £55m on the £365m operating loss for the same period 2011.
The bank said mortgages in arrears are causing big problems - operating profit before impairment losses was £84m, with total impairment losses of £394m.
"This was due to rising arrears and the continued deterioration in asset values as property prices declined further," the bank said in a statement.
"We continue to work with our customers who are in financial difficulty by offering a range of support initiatives."
The bank, which claims 1.9m customers north and south, said it was "making progress" on simplifying its operating model.
"Following a period of consultation and mediation we are now implementing the proposed restructuring plans which we announced in January," the statement continued.
The figures came as parent company RBS revealed a loss before tax of £1.4bn for the three months to March 31, compared to a £2bn loss last year.
RBS boss Stephen Hester however said the bank had met some “important recovery milestones”, and by the end of next week it will have repaid the £75bn in loans it took from the UK government at the height of the financial crisis.