Ireland has been fined €5 million for failing to carry out an EU court-ordered environmental impact assessment on a 70-turbine windfarm at Derrybrien, Co Galway.

The Luxembourg based Court of Justice of the European Union (CJEU) also ordered the State to pay a further €15,000 a day for every day the judgment remains unimplemented.

It warned the farm’s operating licence may yet be removed after the assessment is completed.

The longstanding case brought by the European Commission goes back to a finding by the court in 2008 that Ireland had infringed EU law as a result of the construction of the wind farm at Derrybrien without a prior environmental impact assessment having been carried out.

Although the Irish authorities had since sought to regularise the wind farm’s compliance with the regulations by legislative means, it had not carried out the required assessment.

Such an assessment must take into account not only the future impact of the plant, but also the environmental impact from the time of its completion.

In July, the court’s advocate general had urged it to find against Ireland. “Ireland has confined itself to submitting various proposals...without implementing any of them,” he said. These delays, “despite claims made by Ireland... are not due to a lack of co-operation on the part of the Commission”, he said.

The wind farm was regarded as one of the largest in the EU when a landslide occurred during its construction in October 2003 by an ESB subsidiary, Hibernian Wind Power.

At the time, large areas of forest and peat up to a depth of 5.5 metres on the top of the Cashlaundrumlahan mountain had been removed, causing the 2km-long “environmentally devastating” slide. Fish were killed and waterways polluted when half a million tonnes of peat and debris was displaced.

In a statement after the ruling, the Department of Housing, Planning and Local Government said it “is examining the CJEU ruling”.

Martin Collins, spokesman for the Derrybrien Landslide Action Group, said they were “delighted” with the judgment.

“It vindicates our position over the years and the decades at this stage, it justifies all the time and effort that we’ve put into it, it has been just over 20 years overall, since the 2008 we’re 11 years on, nothing has been done and it’s a complete failure on the part of State agencies and the Irish authorities,” he told RTÉ.

The decision has been welcomed by the Environmental Pillar, a coalition of 28 environmental organisations.

The “ruling marks not just a further stain on the Irish State’s environmental record, but constitutes yet another landslide in terms of the criticism levelled at the State’s handling of this case, and its failures to understand and apply what needed to be done following on from the original judgment,” it said.