“They are not all strategic defaulters, but a reasonable percentage of them would be.” said Mr Brown.
On Thursday, the chief executive of AIB, David Duffy, said that roughly one in five of his customers in arrears were in strategic default.
But whereas AIB saw an increase in mortgage arrears over the second quarter of this year, Ulster Bank reported a decrease in mortgage arrears every month over the past year.
Ulster Bank will try to re-engage with customers who are not paying their mortgages, but if they refuse then the bank would start legal proceedings with a view to repossessing the house, added Mr Brown.
The wholly-owned subsidiary of Royal Bank of Scotland reported €450m in total income for the first six months of the year and an operating profit of €174m before impairment charges.
However, an impairment charge of €503m — which was down from €717m for the first six months of 2012 — pushed the bank into an operating loss of €329m.
The net interest margin was 1.85%. Mr Brown said the target net interest margin was 2.5%-3%.
There was a total loanbook of €32.9bn at the end of June and loan impairment provisions of €4.4bn at the same date. The loan to deposit ratio was 123%.
Mr Brown welcomed recent legislative changes, such as the new code of conduct on mortgage arrears and the repeal of the Dunne judgment, which made it easier for banks to repossess homes. He would like a credit bureau set up over the medium term.
RBS has had to pump roughly £14.5bn into Ulster Bank since the property sector collapsed in 2008.
There has been speculation about the future of the bank since last June when the British chancellor raised the possibility of splitting RBS into a good bank and bad bank, with Ulster Bank going into the bad bank.
Mr Brown declined to comment on this strategic review.