The failure of cash-strapped Mayo General Hospital to collect fees due for treating private patients led to it being owed as much as €6.3m at the end of 2015.
The hospital, which is in the hometown of Taoiseach Enda Kenny, had not billed insurance companies for €4.2m of this at the time, the report of the HSE's internal audit revealed.
It wrote off €116,763 in patient debt in 2015, some of which spanned previous years.
The auditors found that in some cases the incorrect information about the patient's insurance was taken at the admission process and the required forms were not being signed.
Hospital consultants were also not signing off on forms needed to receive payment from the insurers and in some cases they took 527 days, nearly one-and-a-half years, to do so.
A separate HSE audit of Ballyfermot Advance in Dublin which supports people with addictions found that none of its staff was Garda vetted.
The organisation, which received €825,214 in State funding in 2014, found one of its contractors who received payments of €10,000 a year had no tax clearance certificate.
Bank reconciliations were not performed correctly, the audit said.