The HSE's spending on agency staff has increased by over €15.5m this year.
According to freedom of information figures, the bill for the first seven months of 2019 was just under €200m.
That rose to €215m in the same period this year, with large increases on agency nurses and paramedics.
Tony Fitzpatrick, from the Irish Nurses and Midwives Organisation, says it's not down to the Covid-19 pandemic.
"The HSE has research that clearly tells them they need to stabilise their workforce.
"That requires them to recruit additional staff.
"Instead of using directly hired staff, they were using in excess of 3000 whole time equivalent staff per week in order to support the rosters and maintain the rosters."