The Revenue Commissioners want a new tax rate for the country's highest earners.
They say anyone making more than €80,000 a year should have to pay 43% income tax.
Currently the top tax band is 40%, which kicks in when a single person without children earns over €34,500.
Revenue has based its figures on estimates for the tax take next January.
But Sinn Féin's Pearse Doherty says it has set the income threshold too low.
"There's no doubt about it, you can bring in a lot of money if you introduced a third rate of tax.
"What is being reported in the papers today is introducing it at a rate of €80,000. That's not a level I agree with.
"We've always argued that the starting point for a third rate of tax would be the income above €100,000.
- Digital Desk