The Supreme Court has upheld a High Court order quashing a decision to refuse planning permission for a wind farm in Co Laois.
It dismissed an appeal by An Coimisiún Pleanála and concluded that the commission's performance of its function was "insufficient as a matter of law".
The commission had refused permission on the grounds that the wind farm would contravene the Co Laois development plan.
But the Supreme Court ruled that it had failed to consider whether or not the development plan was in compliance with the Climate Act.
Mr Justice Richard Humphreys in January in the High Court quashed An Coimisíún Pleanála’s decision to refuse permission sought by Coolglass Wind Farm Ltd for the development.
He found the board failed to act in a compliant manner, as far as practicable, with climate objectives set out in the 2015 Climate Action and Low Carbon Development Act.
The Supreme Court found that the evident purpose and effect of the Climate Act is to strengthen the obligations imposed upon bodies, including the commission, and "to make the performance of all their functions consistent with the climate objectives insofar as is practicable".
"That obligation is, in principle, enforceable by a court."
The seven Supreme Court judges in their ruling added that the legislation says all public bodies must follow the act "as far as is practicable" and this allows for "a range of possible outcomes open to the relevant body".
It says this could allow the commission to refuse permission to a renewable energy project because it contravenes a development plan, as was the case with the Coolglass proposal, but only if it has considered if that development plan is consistent with climate objectives.
"It is not apparent that the commission ever engaged with that question in a real and substantive way, and instead the decision and report are phrased in terms that seem to imply that the very fact that the development would contravene the development plan was considered itself a reason to refuse the permission."
The ruling means that the Coolglass Wind Farm proposal goes back to An Coimisíún Pleanála for consideration,
The company behind the wind farm, StatKraft Ireland, welcomed the ruling as "positive and important", saying it provides "much needed clarity on the role of climate law in our planning process".
Environmental campaigners also welcomed the ruling.
Friends of the Irish Environment said: "It confirms that planning authorities and all relevant bodies covered by the Climate Act must, insofar as practicable perform their functions in a manner consistent with rapid reductions in greenhouse gas emissions, and with adapting to climate change."
Oisín Coghlan, who campaigned for the 2021 Climate Act, said: "this is not just a procedural nicety - the Court finds it is about outcomes that actually contribute to the law's objective, namely reducing polluting emissions."
He added that he believes it could have consequences for other areas of public policy: "Today’s judgement also seems to contradict the CRU’s [Commission for Regulation of Utilities] position that it doesn’t have authority to regulate emissions from large energy users such as data centres.
"In fact, under the climate law it has a duty to use its existing powers in a manner consistent with reducing emissions. That seems to me to leave its recent positions on data centres wide open to legal challenge."
