The HSE spent nearly €7 million on private ambulances in the first eight months of this year.
This is a sharp increase on the €4.3 million 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.
Therefore, the HSE contracts a number of private companies to assist with deal with less urgent cases.
New figures show the executive spent a total of €6.95 million on external ambulances in the year up until August.
Letterkenny University Hospital had the largest bill, at just over €1 million.
Lifeline Ambulance Service is one of the private providers the HSE employs.
David Hall, the owner, said his company got €2.4 million from the HSE in first eight months of this year.
However, he insists it is not a waste of state funds.
He argued: "Use of private ambulance services nationwide will always be good value for money, based on the need.
"In the absence of this service, the health service would come to a standstill."
Sinn Féin's health spokesperson Louise O'Reilly disagrees.
Deputy O'Reilly suggested: "That money would be better saved if it was invested into the public sector.
"It's the responsibility of management within the HSE to ensure that they are investing sufficient money.
"Indeed it's the responsibility of the Minister to ensure that they're investing sufficient money in directly employed labour and keeping the ambulance fleet upgraded."
The HSE's spend on private ambulance has gradually increased in recent years, from €4.3 million in 2014 to €8.1 million in 2017.
