The HSE spent over €215 million on agency staff in the first seven months of this year.
It's €15.5 million more than in the same period in 2019.
These new figures have been released to Newstalk by the HSE under the Freedom of Information Act.
They show that, by the end of July last year, €199.6 million had been spent on agency staff.
But by the same time this year, that had risen to €215.2 million.
The spend on nurses increased by €2.3 million, and by slightly more for paramedics.
Tony Fitzpatrick, from the Irish Nurses and Midwives Organisation (INMO), says 800 agency nurses are being employed on a full-time basis every week.
He said: "We are concerned over the level of agency usage - it's going in the wrong direction.
"We've highlighted this to the HSE continuously, but despite that the figures continue to increase.
"It's the time the HSE put in place a proper funded workforce plan for the health service to reduce the reliance on agency, which is costing them more than if they directly employed the staff."
When compared to the same period last year, the biggest increase in 2020 was on management and administration staff.
€22.3 million was spent on that by July - over €5 million more than the same period last year.
The HSE says agency spending is under 'constant review', and that reducing and controlling the costs is a 'particular focus'.
They said: "In 2020 both Acute and Community operations have sourced additional agency staff primarily to support the expansion of the workforce in response to the COVID-19 pandemic and also to support service delivery and COVID-19 response due to increased staff absence due to COVID-19.
"Furthermore the HSELive service has been providing the COVID-19 helpline throughout the pandemic and has procured administration support services from a number of external call centres due to the high volume of calls.
"A number of services have also been provided to Testing and Tracing to support the administration of their services and the provision of results to patients."