The HSE spent nearly €1.3 million on debt collection costs in the space of two years.
According to Freedom of Information documents released to Newstalk, the money was paid to six separate companies.
Patients who attend public hospitals normally have to pay certain charges to the HSE.
They include €100 for attending an emergency department and €80 for an overnight stay.
The HSE says hospitals are required to refer an unpaid debt to a collection agency after a period of time.
New figures show the HSE paid six of these companies over €687,000 in 2019 and nearly €569,000 last year.
One company - called Legal and Credit Management Services - got over half a million euro over the 24 months.
Stephen McMahon, from the Irish Patients Association, is questioning the overall cost.
He said: "€1.3 million would pay two or three senior HSE executives' salaries for a year.
"We have to look at really what is the value of spending that money in collecting unpaid amounts from patients that owe the health system money."
The HSE says it tries to recover income in a socially responsible, ethical and cost-effective way.
