The Health Service Executive (HSE) spent over €318m on temporary agency staff in 2018.
The figures were obtained as part of a parliamentary question by Sinn Féin's health spokesperson Louise O'Reilly.
She said the amount of spending was a "sickening waste of money".
"The continuous spending of hundreds of millions of euro by the HSE on temporary agency staff is outrageous; it is a sickening waste of money and is a threat to the State's finances.
"2018 saw the single biggest increase in agency spending since 2014.
"The spending total jumped by €26m on the previous year to a staggering €318m for 2018.
"Since 2011 Fine Gael have spent almost €2bn on temporary agency staff to fill staffing gaps because they have continuously failed to address the recruitment and retention crisis."
"The over-reliance on the use of costly and more expensive agency staff is a direct result of the escalation of the recruitment and retention crisis under this Government."
The figures show Our Lady of Lourdes Hospital in Drogheda spent €9m on agency staff, the Midlands Regional Hospital Portlaoise spent over €10m, while University Hospital Limerick spent more than €11m.
In its response, the HSE said: "This expenditure should be considered in the overall context including increasing demand for services; the impact of earlier constraints on recruitment in the public service; ongoing challenges in relation to the recruitment and retention of clinical staff; actions necessary to support compliance with the European Working Time Directive and efforts to reduce expenditure on agency staff.
"Agency pay cost is under constant review. Agency staff are used where there is a difficulty in recruiting and employing hospital staff and where there is a short-term critical service need.
"Agency is also used for flexibility to allow for variation in activity and as required to meet patient demand needs."