The UN Permanent Court of Arbitration has ordered the British government to consult with Irish officials before carrying out any future reprocessing at the Sellafield nuclear plant.
The interim order was made by the court last night in advance of a full hearing of Ireland's case against Sellafield.
Britain was also ordered to devise a plan for further co-operation with Ireland on nuclear safety issues.
The Irish Government has argued that discharges from Sellafield and the threat of an attack on the plant pose an unacceptable health risk to the Irish population.
The UN court is due to give its final ruling on the matter later this year.