The HSE spent nearly €200m employing agency staff over a seven-month period.
An average of €28m a month was spent on paying doctors and nurses from agencies since the beginning of the year. The total cost for 2016 so far now stands at €197,931,000.
Agency costs for the year were lowest in January, with €27,349,000 spent by the health service.
Meanwhile, July proved to be the most expensive month, with €30,034,000 paid.
The figures were released in a parliamentary question by Dublin TD Tommy Broughan. He said he raised the question after meeting with members of the Irish Nurses and Midwives Organisation, SIPTU and the Psychiatric Nurses Association, and called the amount spent "staggering".
"Averaging just over €28m per month, it is mind-boggling to think that this short-sightedness is limiting long-term resources for our health service," he said.
"In many services, in particular residential services for those with disabilities, continuity of care and establishing relationships is vital to the well-being of the patient or service user.
"A large portion of this money would be much better spent hiring, training, maintaining and properly remunerating public service staff."
Earlier this year, it was revealed that the HSE had spent almost half of its €226m agency staff budget for 2016 by the end of April.
The HSE has previously insisted that the use of agency staff is closely monitored and only used when "strictly necessary" in order to prevent service disruption.
Directly-employed staff account for around 95pc of the HSE's employment costs.
The largest element of the extra cost for agency staff is the 23pc in Vat, which goes to the exchequer.