The HSE spent more than half-a-billion Euro on agency staff in the first 11 months of last year.
Freedom of Information figures released to Newstalk show the health service spent at least 32% more on agency staff last year than in 2020.
In 2019, the HSE spent €349m on agency staff - rising to €390m in 2020.
That then increased to €513 million in the first 11 months of last year.
The spending included €162m on support services, €110m on nursing staff and €98m on management or admin staff.
UCC health economist Brian Turner said some of the increased spend is COVID-related.
“Things like contact tracing, the HSE Live phoneline – those kinds of areas, agency staff were needed to staff those, which is perfectly understandable because obviously we hope they are very temporary.”
Beaumont Hospital Consultant Professor Bill Tormey told Newstalk the figures are a concern.
“The fact the HSE was able to pay people over the odds and fund bodies to do certain jobs makes you think what is going on all the time with staff shortages etc,” he said.
“The reason they are able to do this and get staff is people opt out of the public service and become privateers and become either agency workers or work for themselves.”
He said many HSE staff quit the public sector to become agency workers.
“The question you have to ask yourself is, why do people opt out of the public service and why are people unhappy in certain circumstances and will do the same thing for more money,” he said.
“It is the old story of money talks and, if there is a monopoly as there is with the HSE, what are they doing to actually counter this and the answer, obviously, is very little.”
In a statement, the HSE said extra agency staff were sourced in 2020 and 2021 to assist in the pandemic response.