I believe the Deputy may be referring to the Revenue Commissioners' Studies "Effective tax rates of the top 400 earners: Report for the tax year 2001" and "Effective tax rates of the top 400 earners: report for the tax year 2002", which were published in March 2005 and June 2006 respectively.
The Revenue Commissioners in paragraph 4 of the report for the tax year 2001 set out changes in the methodology to be applied in such studies in quantifying and classifying tax paid. The changes were that: deposit interest retention tax (DIRT) deducted at source from any deposit interest earned was to be included in the figure for income tax payable in all cases in computing the effective tax rate; and individuals paying some income tax, although amounting to an effective rate of less than 1%, were to be excluded from the 0% range and included in the less than 5% range.
An appendix to the report for the tax year 2001 showed the effect of these changes which was relatively minor. That appendix then provided the comparative figures when the new methodology was applied in the report of the Revenue Commissioners on the effective tax rates of the top earners for the tax year 2002.