People claiming the Covid-19 unemployment benefit are risking the prospect of big repayments if they work even a couple of hours a week.
Concerns have been raised that some recipients may not realise they must close their claim, even if not working full-time.
This may apply to part-time or self-employed workers who return to work on reduced hours, or are contracted to carry out individual projects.
Unlike standard jobseeker benefits, those on the pandemic payment cannot work part-time and collect benefits for the days they do not work.
There are almost half a million people on the pandemic payment - although the number fell from a peak of almost 600,000 in May.
"The pandemic unemployment payment is only payable if an employee has lost their job and is fully unemployed," said a spokesperson at the Department of Employment Affairs and Social Protection.
"People who return to work but on reduced hours may be eligible for other income supports depending on the individual's circumstances."
They said these included jobseeker's allowance or benefit, which can be paid to people working up to three days a week. They could also apply for short-time work supports or a part time-job incentive scheme.
The department said it used a variety of techniques to detect suspected fraud and non-compliance. "One such mechanism is by working collaboratively with gardaí, other government departments and agencies," it said.
Those who were earning up to €199.99 a week prior to the crisis are already due to have their pandemic payments reduced to the standard €203 a week jobseeker rate from June 29.
However, those who earned €200 or more - and make up three-quarters of recipients - will remain on the €350-a-week rate.
"People may be under the misapprehension that they can do a little bit of work and would want to be very careful, because if it comes to light they might be facing a substantial overpayment bill," said Fianna Fáil social protection spokesperson Willie O'Dea.
"The pandemic payment was a simplified payment that had to be brought in quickly. Otherwise, it would have been too hard to manage and you would have needed an army of civil servants to manage it."
Head of policy and media at the Irish National Organisation of the Unemployed Bríd O'Brien said there was no flexibility with the pandemic payment.
She said someone could "get a land" if they came off the payment and found they had not made enough PRSI contributions to qualify for jobseeker's payments.
They might have to apply for a means-tested payment and if they were living at home with parents, or with a partner, their incomes would also be taken into account.
Laura Bambrick, of the Irish Congress of Trade Unions, said "the ideal" scenario for a person who takes up part-time work would be to transfer to the temporary wage subsidy.
However, their employer would have to be signed up to the scheme, managed by the Revenue Commissioners.
This pays 85pc of their previous weekly average take-home pay, up to €410 depending on their earnings, with a potential top-up from their employer.