Taxback has called on Revenue to address the "bizarre" anomalies in the flat-rate expense allowances so that all workers get their fair dues.
The flat-rate expense allowance enables certain workers to cover some of the costs incurred in carrying out their work.
Taxback said that many workers are unfairly losing out on a tax allowance worth hundreds of euro, and in some cases thousands, a year, due to anomalies around the allowance.
The discrepancies mean that some workers are only entitled to claim half the tax allowance of their counterparts, and that particular occupations and professions are entitled to much more generous allowances than others.
The anomalies include female cardiac technicians being able to claim a higher annual tax allowance than their male counterparts - €212 compared to €107.
Doctors, nurses, opticians and veterinary surgeons can claim back the cost, or a portion of the cost, of mandatory registration fees for their professions, but veterinary nurses cannot.
Nurses who supply and launder their own uniforms can claim an annual allowance of €733, but nursing assistants only get an allowance of €526.
Meanwhile hospital kitchen porters have to make do with €360 as do hospital cooks, catering supervisors, waitresses and catering staff.
Others who receive a much smaller annual allowance than nurses for the supply and laundry of their own uniforms include cosmetologists (€160), occupational therapists (€217), physiotherapists (€381) and veterinary nurses (€400).
Taxback also said that waiters are not specifically entitled to a tax allowance for the supply and laundry of uniforms and the general annual flat-rate expense allowance of €80 is all they will get to cover this cost.
Marian Ryan, consumer tax manager with Taxback, said that flat-rate employment expense allowances can be valuable for workers.
"For unknown reasons, there are bizarre discrepancies in these allowances which mean many workers perhaps unfairly get better allowances than others, while some workers lose out entirely," she said.
She said it is now more than four years since Revenue conducted a review of flat-rate expenses allowances.
"Any planned changes to the allowances had been deferred until January 1, 2023, but Revenue recently said that a further review is now required, given the passage of time since the initial review," she said.
"There is no time for any further delays - Revenue must now make the overhaul of flat-rate expense allowances is priority," she said.
"Given the huge inflation of recent years, this allowance will go a long way for many workers so it is important that more are given the opportunity to claim it - and that any unfairness in allowance rate is eradicated," she added.
Ms Ryan also said that she believes office workers should be able to claim the flat-rate expense allowance, regardless of worker category, to reflect the office clothes and other costs they incur in the course of their work.
"Not all office workers are currently entitled to the allowance, even though they make up a large portion of the working population," she said.
"A flat-rate expense for e-workers should also be considered to replace the remote working relief which currently must be claimed by remote workers. Remote working relief is an underclaimed relief," she noted.
