The Department of Finance has said the Revenue Commissioners are carrying out an analysis of the top 400 income earners.
It said that when the research is completed the department would be in a better position to say why some top earners paid little or no income tax in 2001.
The announcement came after the Labour Party claimed that 41 people or couples with incomes of more than half a million euro paid no tax in the year 2001 - including 11 people or couples with incomes of more than €1 million.
The department said there could be several reasons why some people have no tax liability, including tax relief for pension contributions or tax breaks, which were introduced to stimulate investment.
