The Supreme Court has quashed the 2017 National Mitigation Plan on the basis that it does not provide enough detail about how the State will reduce greenhouse gas emissions.
The case against the State was brought by Friends of the Irish Environment (FIE).
The landmark decision is likely to have environmental policy implications elsewhere in Europe.
FIE had argued that Ireland had a responsibility to rapidly reduce greenhouse gas emissions within the next couple of years or face runaway climate change.
They argued that the National Mitigation Plan permitted an increase in greenhouse gases and so was contrary to the 2015 Climate Action and Low Carbon Development Act, which required the publication of a plan for transitioning to a low carbon climate resilient and environmentally sustainable economy by 2050 described at the National Transitional Objective.
The unanimous judgment of the Supreme Court was delivered by Chief Justice Mr Frank Clarke.
It ruled that the 2015 Act requires a sufficient level of specificity in the measures identified in a compliant plan to meet that National Transitional Objective by 2050, so that a reasonable and interested person could make a judgment both as to whether the plan in question is realistic and as to whether they agree with the policy options.
The court concluded that the plan falls well short of the level of specificity required by the 2015 Act. On that basis, it quashed the National Mitigation Plan.
Speaking on RTÉ's News at One, Climate Case Ireland spokesperson Clodagh Daly said it was a "groundbreaking and landmark" verdict and thanked those who took the case on their behalf.
She said the verdict means the Irish government "can no longer make promises it will not fulfil" and has a legal obligation to protect citizens from the worst impact of climate change.
Ms Daly said the verdict has "huge ramifications nationally and internationally", and confirms that developed countries must take the lead in reducing emissions.
Oisín Coghlan, Director of Friends of the Earth, said: "The Government already had a scientific, political and moral obligation to step up its efforts to cut climate-polluting emissions.
"Now the Supreme Court has ruled it has a legal obligation as well."
The Supreme Court has quashed the 2017 National Mitigation Plan on the basis that it does not provide enough detail about how the State will reduce greenhouse gas emissions. | https://t.co/Zu8urU3kgL pic.twitter.com/p7UjS6QgOh— RTÉ News (@rtenews) July 31, 2020
Political reaction to the ruling
Climate Action Minister and Green Party leader Eamon Ryan said in a statement: "I welcome the judgment of the court and congratulate Friends of the Irish Environment for taking this important case.
"We must use this judgment to raise ambition, empower action and ensure that our shared future delivers a better quality of life for all."
The statement said the new Government is "now committed to an average 7% per annum reduction in overall greenhouse gas emissions from 2021 to 2030, equivalent to a 51% reduction over the decade and to achieving net zero emissions by 2050".
The Department will now "carefully examine the decision and consider its implications".
Green Party Councillor for Cabra/Glasnevin in Dublin, Darcy Lonergan, campaigned on behalf of the FIE.
She said today: "When we packed the courthouse over a year ago, I never dreamed we would get a unanimous ruling from the Supreme Court. It's not only a win for the climate, but for persistence and dedicated campaigning."
Sinn Féin spokesperson on Climate Action, Darren O'Rourke, said: "When the National Mitigation Plan was published back in 2017, Sinn Féin said it was a rehash of previous commitments, short on detail or new ideas and would not guarantee that the state meets its international obligations on the reduction of emissions.
"The judgment of the Supreme Court today vindicates that position."
He added: "Years have now been wasted, so the Minister must act on this judgment urgently and bring forward a new, detailed and ambitious plan."
Labour spokesperson on Climate Action, Duncan Smith, commented: "Under the 2017 plan, Ireland's climate emissions have actually risen.
"Now the Supreme Court has rightly told the new Government that Labour’s 2015 law requires them to come up with a more robust plan."
Fianna Fáil MEP, Billy Kelleher, who is a member of the EU Parliament's Environment Committee, said: "False starts and badly developed plans only leave us open to greater climate risks and financial fines.
"Irish society gets the need to make the change to a low-carbon economy. Let's get detailed plans agreed and let’s get to work. We don’t have any more time to waste."