SIPTU says between 15-hundred and two thousand health workers who signed up to be "On Call for Ireland" were handed agency contracts with inferior terms and conditions.
The trade union says it's been contacted by a number of workers hired under the scheme.
It says the contracts don't include death-in-service benefits which could amount to up to two years' salary for a worker with a direct state contract, or Covid-19 leave pay.
SIPTU's Paul Bell says agency staff are taking all the same risks - and should get the same treatment.
"These workers because they're not in that pension scheme have no access to any type of cover if they become a fatality, and please God that doesn't happen.
"We don't want to see a sit-in where a healthcare worker in that type of contract succumbs to an illness of Covid 19, and then their family or loved ones have to pursue the state or the employer for some kind of compensation."
