According to Freedom of Information (FOI) figures, the bill for the first seven months of 2019 was just under €200 million.
That rose to €215 million in the same period this year, with large increases on agency nurses and paramedics.
Tony Fitzpatrick, from the Irish Nurses and Midwives Organisation, says the increase is not down to the Covid-19 pandemic.
"The reason is that the HSE has been going in the wrong direction in regards usage. They have research that clearly tells them that they need to stabilise their workforce.
"That requires them to recruit additional staff, which they have failed to do. Instead of using directly hired staff, they are using in excess of 3,000 whole time equivalent staff per week in order to support and maintain their rosters."
