The Court of Justice ruled that assessments should have been carried out on the Derrybrien wind farm site before the project was given the go-ahead.
A landslide at the eventual farm, in 2003, led to the death of 50,000 fish.
Derrybrien, the largest wind farm in Ireland, will now set a precedent in how the government grants future projects.
Authorities will now have to ensure all environmental checks required under EU law are carried out.
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The 2003 landslide at Derrybrien after 450,000 cubic metres of peat dislodged over a 32km area, polluting a river and killing 50,000 fish.
At the time the government blamed poor construction work, but the court found that it was because a proper environmental impact assessment or EIA had not been carried out.
The court's ruling covers other cases where an EIA was done only after works began. These include quarry developments in Offaly, Galway, Waterford, Clare and Monaghan.
Similar assessments also relate to pig-rearing, peat-extraction and wood-processing enterprises in other counties.
A hotel in Kildare, which received retention permission only after it was built, is also subject to the ruling. As is the Citywest convention centre, where work began without proper planning approval.
The European Court ruled that retention permission can be applied only in exceptional circumstances, and raised the point that the current application of Irish law may be too loose.
(PR/JM)
