The HSE spent over 1.5 million euro a day on agency staff last year.
The bill for the first 11 months of 2021 was 513 million, compared to 390 million in all of the previous year.
Beaumont Hospital consultant, Bill Tormey, says many 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?
"It's the old story of money talking, and if there's a monopoly - as there is with the HSE - what are they doing to counter this?
"The answer is obviously very little."
The HSE says extra agency staff have been needed during the pandemic - partly to cover for employees who have the virus.
But Tormey says the figures are a concern.
"The fact that the HSE was able to pay people over the odds and find bodies to do certain jobs, it makes you think what's going on all the time with staff shortages."
