The increased income levies introduced in this month's emergency Budget are reportedly set to be backdated to the start of the year.
When announcing the budget, Finance Minister Brian Lenihan said the tax and PRSI changes would come into force on May 1.
However, reports this morning say it will now be backdated to January 1.
The move could reportedly affect lump-sum payments such as bonuses which have been paid to workers in the first four months of the year.
This morning's reports say it will also catch business owners and company directors who were urged to "front-load" their 2009 income ahead of the Budget to avoid higher taxes.