The High Court has ruled that it cannot "second guess" the Government's climate policy.
The court recognised the severity of the climate crisis - but ruled that Government policy is a matter for elected officials.
The case, taken Friends of the Irish Environment, argued that the National Mitigation Plan falls far short of what is needed to tackle climate change.
It comes as thousands of Irish students are set to join their counterparts around the world in leaving class to strike for climate action tomorrow.
Spokesperson Sadhbh O'Neill said the group will now consider the verdict - but the fight will go on.
"We are disappointed but it's not the end of the road - we have many, many challenges facing us.
"We need to go out there tomorrow and support all the young people striking, because climate change is still there, it's still a problem we have to confront.
"We may use litigation one day and we're going to have to get our bodies out on the street the next day.
"One way or the other, we have to drive home the message to our political leaders that they need to take urgent action".