In one case, an OPW investigation into discrepancies between the number of tickets sold at one of the sites and the cash lodged between 2004 and last year revealed that over 11,000 was missing.

The heritage site involved was only open for 44 weeks during that period.

In the second separate and unrelated case, the OPW was alerted when an unusually large amount of cash was lodged last year.

Subsequent investigations into returns from the site over the last three years revealed that over 30,000 was missing.

It is understood that on both incidents, the visitor guides employed at the two sites on a casual basis were interviewed by gardai.

In one of the cases, the visitors' guide had made some efforts to pay back some of the money. This did not happen in the other case.

A spokesman for the OPW confirmed that gardai are still investigating the two incidents of the missing cash, but no files have been sent to the DPP as yet.

The incidents first came to light in the Comptroller and Auditor General's report for 2006, which was published in September.

The Comptroller, John Purcell, said that a common theme emerging from the two incidents was the "failure to lodge moneys on a weekly basis as required and the failure of the Visitors' Services Section [of the OPW] to detect this in a timely manner."

However, steps have been taken since the two incidents to significantly increase the staffing in the visitors' section.

This will enable the necessary levels of scrutiny and control to be consistently and properly applied, according to the Comptroller.

The OPW manages 747 heritage and 19 historic sites around the country, of which 89 are open to the public on a seasonal basis. Of those 89, the public are charged admission in 50 sites.

In 2005, income from these sites brought in 5.6m. This figure increased to 6.5m last year.