The government will now be forced to redraft its plan.

The legal challenge, brought by Friends of the Irish Environment, contended a 2017 government climate action scheme breached legislation requiring the state to specify its proposals.

In his written finding, chief justice Frank Clarke said there was "a clear present statutory obligation on the government, in formulating a plan, to at least give some realistic level of detail".

"In my judgement the plan falls a long way short of the sort of specificity which the statute requires," he added -- quashing the plan on the grounds it failed to meet its "statutory mandate".

During proceedings the government argued that the legal challenge amounted to a challenge of its policy decisions which are "not within the scope of questions which can properly be the subject of litigation".

The government has also called its 2017 plan a "living document" which does not "provide a complete roadmap to achieve the 2050 objective, but begins the process".

However Clarke said "the public are entitled to know how it is that the government of the day intends to meet" the 2050 commitments.

Climate minister Eamon Ryan -- who led the Green Party into a new coalition government last month -- welcomed the judgement and said it reflected "the importance of climate change as an existential challenge to humanity".

"We must use this judgement to raise ambition, to empower action and to ensure that our shared future delivers a better quality of life for all," he added.

In 2015, legislation committed the government to produce plans for "the transition to a low carbon, climate resilient and environmentally sustainable economy by the end of the year 2050".

Ireland has also committed to reduce carbon dioxide emissions by 80 percent between 1990 and 2050 across the electricity generation, built environment and transport sectors.