The Government is making another attempt to have the Sellafield nuclear plant in Cumbria shut down permanently.
A legal team led by Attorney General Rory Brady will claim at the UN Convention on the Law of the Sea, in The Hague, that radioactive discharge from the Sellafield site is polluting the Irish Sea.
He will argue that the Irish Government's protests were not taken into account when the licence for the Cumbrian complex was granted.
The Minister for the Environment has also expressed concern at the inadequacy of the environmental assessment undertaken by the UK in relation to the facility and the failure to properly assess the risk of terrorist attack on the site.
Martin Cullen has described today's action at a United Nations court as essential to protect Ireland's interests. He said he regretted that such a step was necessary.
The British government has rejected claims that discharges from the MOX plant pollute the Irish Sea. It is expected their legal team will argue that the court does not have jurisdiction over Sellafield.
The hearing, which will take place before a five-member tribunal, is likely to last until the end of the month.
