The HSE spent nearly €4.5 million on private ambulance costs in the first six months of this year.
A further €14.3 million was also spent on taxis in the same timeframe.
The National Ambulance Service's main role is to help people with serious or life-threatening conditions.
Private companies are often employed to deal with less urgent cases, such as transfers between hospitals.
According to Freedom of Information figures released to Newstalk, the HSE spent nearly €4.5 million on private ambulance costs in the first six months of this year.
Letterkenny University Hospital's bill was the highest, at over €629,000.
Mayo University Hospital spent €414,000.
One provider - Lifeline Ambulance Service - got over €1.4 million in total.
David Hall is that service's chief executive, and he says it's not a waste of money.
He said: "The spend that the HSE spends is probably the most efficient expenditure for ambulance service and transport that is available throughout Europe.
"We don't get paid unless we move a patient... we don't get paid to stand-by.
"They don't pay a premium, there's no overtime, and they don't pay for cancellation of calls."
The HSE also spent €14.3 million on taxis in the first six months of this year.
It says taxis are often used where an ambulance is not needed, such as transferring dialysis patients from home to acute hospitals.