The HSE has paid €15.5m to private recruitment firms since 2020 but does not know how many staff have been hired as a result.
The amount paid to recruitment agencies jumped from €2.88m in 2020 to €9.78m last year. That included payments of more than €2.5m each to two companies.
A further €2.6m has already been paid out in the first five months of this year.
Labour TD Sean Sherlock has accused the HSE of continuing to pay millions of euro of taxpayer funds to private companies with no clear return on investment. He said it is "frustrating and mind-boggling".
"We need to grasp the nettle in respect of the recruitment issue. For two years now we are being told the issue is recruitment and that we cannot recruit the staff," he said.
"I do not think the public are buying that anymore with over €15 million spent on companies to recruit," he said.
A breakdown of spending in each of the HSE regions shows that the North West and Mid-West (NW MW) finance area, which takes in Donegal, Sligo, Leitrim, west Cavan, Clare, Limerick and north Tipperary, spent €1.36million on recruitment firms in 2020. That was by far the most of any region. That spending rose to €7.36m last year, while €645,231 had been paid out up to June of this year in NW MW.
The HSE cited the outsourcing of certain recruitment services to TTM Healthcare in relation to Manorhamilton, Co Leitrim as well as additional staff requirements in the intensive care and high dependency units at Limerick Hospital as a reason for this spend.
More generally, Winter Plan initiatives were given as an explanation for the increase in costs in 2021 compared to 2020.
The HSE said information relating to the number of staff that have been hired through recruitment agencies is "not centrally available" as it is held in "disparate HSE Regional Finance Ledgers".
A spokesperson said gathering information on the amount spent across the country on private recruitment firms is "time consuming and resource intensive".
But they said the funds were paid out for recruitment advertising costs, placement fees and some agency pay.
Responding to parliamentary questions posed by Mr Sherlock, the HSE said: "It is not possible to identify within the finance systems the specific areas for which the recruitment firms were recruiting other than to provide the finance region from which the payments were made.
"The shortcomings in the HSE legacy financial systems are well acknowledged and their replacement by a single standard financial system for the health sector is at the core of the Finance Reform programme initiated by the Department of Health."
Among the firms used by the the HSE in 2021 was Kate Cowhig International Recruitment which received €2.68m and TTM Healthcare which was paid €2.55m. CPL Solutions was paid €665,592 last year, CPL Healthcare received €553,910 and Tappa Holdings, trading as Red Group got €381,204.
Mr Sherlock said the spend on recruitment firms needs to be drilled down into and justified.
"The Government, through the HSE, has spent €15m in the last two years giving money to recruitment agencies. What are they doing for that money? That is the question. The minister cannot just wash his hands of this."
He added: "We have been quite patient and people have understood that there is a recruitment issue but it is time for us to start drilling down and getting into the weeds of why we cannot recruit staff in a way that meets the demand and the needs of people who are affected."