A hospital with long waiting lists and emergency overcrowding was owed €25.5m at the end of last year by private health insurers, the Comptroller and Auditor General has found.
The funds, which were due to University Hospital Galway, were part of nearly €300m owed by health insurance companies for the treatment of private patients in public hospitals at end of 2014.
The audit found the Exchequer is meeting the funding gap while the money remains unpaid.
It found consultants are still slow in signing the necessary paperwork to claim the money from the insurers and the delays are getting longer.
It can be as long as 163 days in some cases before they sign the forms.
The doctors are always asked to explain the delays.
Insurance companies are also querying more claims, following changes which charge insured patients even if they are placed in a public bed.
The report found hospitals ended up writing off €4.6m owed in private patient fees last year.