Legal action over emissions from the Sellafield nuclear plant in Cumbria breached EU rules, a judge has said.
The row centres on coastal emissions from the Sellafield plant
The Irish government launched legal action at the United Nations in 2001 over alleged marine pollution of the Irish Sea from the West Cumbrian site.
But the European Court of Justice says Dublin should have asked the European Commission to settle the row.
However, Irish lawyers insist they were right because of the alleged flouting of UN environmental protection laws.
The European Commission argued the row over Sellafield was a matter which could and should be tackled within the EU.
Wednesday's opinion is not a final verdict. That will be delivered later this year by the full court.
Advocate General Miguel Poiares Maduro said Ireland had breached its "duty of cooperation" under EU law by bringing in the UN.
Sellafield's controversial mixed oxide plant (Mox) recycles plutonium from spent nuclear fuel from all over the world.
It was constructed by British Nuclear Fuel following an environmental impact study published in 1993.
The plant was completed in 1996, but authorisation to operate it only came in October 2001, after five public inquiries into its economic justification.
Less than a month later, Ireland took its complaints to the UN body, citing environmental and health concerns over emissions and complaining the UK had failed to provide Dublin with a copy of the report assessing the plant's economic justification.