The HSE spent nearly €7 million on private ambulances in the first eight months of this year.
This is a sharp increase on the €4.3m spent in all of 2014.
The National Ambulance Service responds to emergency calls 24 hours a day but often struggles to meet the level of demand.
The HSE contracts a number of private companies to assist with deal with less urgent cases.
New figures show the HSE spent just under €7m on external ambulances in the first eight months of this year.
Letterkenny University Hospital had the biggest bill at just over €1m.
Lifeline Ambulance Service is one of the private providers the HSE employs and David Hall, the owner, said his company got €2.4m from the HSE in first eight months of this year.
"Use of private ambulance services nationwide will always be good value for money based on the need," said Mr Hall.
"In the absence of this service, the health service would come to a standstill."
Sinn Féin's health spokesperson Louise O'Reilly disagrees saying that the money spent would be better used if it was invested into the public sector.
"It is the responsibility of management within the HSE to ensure that they are investing sufficient money and indeed it is the responsibility of the Minister to ensure that they are investing sufficient money in directly employed labour and keeping the ambulance fleet upgraded," said Ms O'Reilly.
The HSE's spend on private ambulance has gradually increased in recent years, from €4.3m in 2014 to €8.1m in 2017.