An Garda Síochána visited the Sellafield nuclear reprocessing plant in June to see the physical security measures in place there to prevent against terrorist attack.
As part of an official agreement between the British and Irish governments to share information on nuclear issues, officers were given a tour of the facility and a briefing on security at the site by members of the Nuclear Installations Inspectorate and its dedicated nuclear police force.
However, the gardaí were not given details of all the security measures and risks regarding the potential terrorist threat against the site, one of the highest profile civilian nuclear installations in Europe.
A report was prepared for the Government on the visit, but is not being made public for security reasons.
The visit was part of a new agreement between the British and Irish governments on the sharing of information relating to nuclear issues, which was signed yesterday by the Minister for the Environment, Mr Roche, and the British Ambassador, Mr Stewart Eldon, during an official ceremony at the Custom House.
The signing ceremony also marked the first time in more than 83 years that the Union Jack flew at the Custom House.
The last occasion was May 5th, 1921, the day the building was severely damaged by fire during the War of Independence.
Under the agreement, both countries are committed to co-operate fully in the event of a significant nuclear incident, and it has already led to a number of initiatives.
It followed the case taken three years ago by Ireland against Britain to the UN's tribunal for the law of the sea to try and prevent the opening of the mixed oxide nuclear fuel plant.
As part of the case, the tribunal recommended enhanced co-operation between both countries on nuclear issues.
There are now formal arrangements for access to Sellafield by the Radiological Protection Institute of Ireland (RPII), which visited the site to evaluate the potential environmental risks of the waste stored on the site, and took part in an emergency exercise which simulated a major incident at the plant.
The RPII also has direct access to all monitoring data in Britain relating to radiation levels around the country.
Mr Roche said there was "an extraordinary degree of openness" on the part of British authorities during the Garda visit, led by an assistant commissioner, in June.
"I think it's comforting that the visit took place and it represents significant progress."
He said the British government could not have been expected to provide all security details as it was "a sovereign issue".
Welcoming the agreement, Mr Roche rejected suggestions the Government had softened its position towards Sellafield.
"The signing of this agreement does not signal a change in our policy," he said.
"We remain committed to the Programme for Government, which calls for the safe and orderly closure of Sellafield."
The British ambassador, Mr Eldon, said the agreements were wide ranging and went "beyond what either party envisaged at the start of the discussions" 18 months ago.