Friends of the Irish Environment argued that the government's climate action plan breached human rights obligations and contravened Ireland's constitution.
Judge Michael MacGrath said Thursday at the High Court in Dublin that it was "a very complex case."
He said the government argued that its National Climate Change Mitigation Plan was only "an initial step in making the country a low-carbon economy," while activists said "there is no hope of achieving this and the plan is inadequate."
The judge concluded that the plan was not unconstitutional.
He said the government had wide discretion for its policies and it wasn't for the court to decide on them.