The HSE spent nearly 4.5 million euro on private ambulance costs in the first six months of this year.
A further 14.3 million euro was also spent on taxis in the same time frame.
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, the HSE spent nearly 4.5 million euro on private ambulance costs in the first six months of this year.
Letterkenny University Hospital's bill was the highest, at over 629,000 euro.
Mayo University Hospital spent 414,000 euro.
One provider - Lifeline Ambulance Service - got over 1.4 million euro in total.
David Hall is its chief executive - he says it's not a waste of money.
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.