The Irish government has been granted unprecedented access to the Sellafield nuclear processing plant in Cumbria as part of a unique partnership.
Sellafield is on the west coast of England
The Radiological Protection Institute of Ireland (RPII) and Irish police will be allowed to visit the site.
The agreement comes after legal action brought by Ireland.
There have been fears from the Irish government that Sellafield radiation may be responsible for poor health in some people on the east coast.
The east coast of the country lies across the Irish sea from Sellafield.
The RPII will also have access to the UK's radiation monitoring system.
Irish environment minister Dick Roche described the agreement as a "positive development in our shared interest in managing our respective positions on the nuclear energy issue".
But Mr Roche stressed that the total closure of Sellafield is the Irish Government's objective.
Stewart Eldon, British Ambassador to Ireland, said: "The governments discussed a wide range of issues to ensure that the system of inter-governmental notification and co-operation is as robust and effective as it can be."
The Irish government has also been increasingly concerned about the catastrophic effect a possible terrorist attack on Sellafield would have on Ireland.