More than €150,000 a week is spent on private ambulances by the HSE - with the overall bill doubling in just four years.
A record €600,000 a month is used to pay a range of private operators, latest figures obtained by the Sunday Independent
Sligo University Hospital spent €634,347 in 2016 for the private service.
The rapidly escalating bill comes as the HSE has warned the Government it could face an unprecedented financial shortfall of up to €881m this year.
Now, official records show the bill paid to private ambulance firms totalled €7.8m in 2016.
In 2015, some €6m was allocated to private companies providing this type of service.
The bill remained unchanged in 2014 and 2013, totalling €4.3m in each year.
In 2012, the figure stood at €3.7m.
In total, the HSE has paid €26m to private firms from 2012 to 2016.
Private ambulances are mainly used to transfer patients, and do not respond to emergency calls.
Rural hospitals have the largest spend on private ambulance services.
Letterkenny General Hospital and Mayo University Hospital allocating €738,981 and €770,943 respectively.
Critics argue a failure to adequately resource the National Ambulance Service has led to an increased reliance on private operators.
In a statement, the HSE said the National Ambulance Service (NAS) was tasked with helping people with "serious or life-threatening conditions".
It added that its main role was to respond to emergency 999 or 112 calls.
However, a "significant requirement" to transfer "non-urgent public patients" for various clinical reasons meant private ambulances were used to provide "additional capacity" to handle such cases.
They are used in a variety of circumstances, including to transfer patients between hospitals, between residential and acute settings, and transfers for diagnostic tests at another hospital. Taxis may also be used in cases where an ambulance is not required.
Meanwhile, new records show 104 formal complaints have been lodged with the National Ambulance Service in the past three years by members of the public.
Forty-five complaints related to the 'attitude and behaviour' of paramedics.
Allegations of assaults, misdiagnosis, incorrect treatment and 'dangerous driving' with a patient on board were among the main issues raised by service users. Lack of discretion when asking patient questions, issues of "consent" to treat patients and allegations of assault were also lodged with the NAS.
Data also shows a total of 47 attacks by members of the public, and service users, on ambulance staff in 2016.
A NAS spokesperson refused to provide the location and nature of each incident.
He said to provide such information would involve the disclosure of "personal information" which could lead to the identification of certain individuals.
Last month, the HSE warned the Government it could face potential financial difficulties of almost €900m this year.
It resulted in Health Minister Simon Harris strongly criticising the executive's "poor budgetary performance".
In a letter to HSE director general Tony O'Brien, Minister Harris said the executive's service plan for the year must be in line with available funding "and not qualified by what the HSE had sought".
Mr Harris said the issue of financial resources had been emphasised strongly a year ago.
He added that he was "extremely disappointed" that in a recent budget submission to the Department of Health Mr O'Brien "did not take this clarification into account".
Mr O'Brien has raised doubts that value-for-money savings of €346m would be achieved in full.
The savings were a key element of the health budget in 2018.
Taoiseach Leo Varadkar has warned the HSE it cannot continue to seek extra Government funding without reforms. He says better management, proper clinical leadership, real accountability and proper responsibility were required from those charged with running and managing the State's health service.
This year, the HSE and the health sector has the largest budget in the State's history, at approximately €15bn.
This represents an increase of €2bn per year in the past three years.