More than 60 public beds in Beaumont Hospital in Dublin are occupied by private patients, according to an internal report.
But the hospital is not allowed to charge health insurers such as the VHI when it puts their members into public beds, according to the report by the hospital's chief executive, Mr Pat Lyons.
At the same time, a number of private beds are occupied by public patients at any given time. This arises from the need to isolate patients with infections when the only single rooms available may be private.
Public patients may also be moved to private rooms when they are terminally ill.
Sometimes public patients admitted through the accident and emergency department are allocated private beds when no public beds are available.
One effect of this is that last year private beds were occupied by private patients only 66 per cent of the time.
Mr Lyons told The Irish Times that as many as a third of admissions through the accident and emergency department are of private patients.
If there is no private bed available, they must be given a public bed - in the same way that a public patient may be allocated a private bed if there is nothing else available.
The number of such admissions of private patients through accident and emergency has increased in the past two to three years.
It is thought that the increased number of people joining group insurance schemes at work as a result of the buoyant economy is partly responsible for this.
Beaumont has 106 VHI-approved private in-patient beds but could fill 166 private beds, Mr Lyons's report to the board says.
He says the VHI approves only two private day-beds in the hospital.
"This does not reflect the substantial growth in day case activity and the continued move of resources to treatment delivery through day facilities," he notes.
The hospital gets more than £5 million of its annual income from its charges to insurance companies for private beds.
The amount it can charge for private beds is regulated by the Department of Health and Children. This year the charge is £183 a day for a private bed, £143 for a semi-private bed and £131 for a day-care bed.
These represent increases of about 7 per cent on last year's charges.
The increase will bring in an extra £453,000 for the hospital this year from insurance companies, but its allocation from State funds is reduced by the same amount so that, overall, there is no gain to the hospital.