Green Party TD Patrick Costello has won his challenge to Ireland's proposed ratification of the Comprehensive Economic Trade Agreement trade deal between the European Union and Canada.
The High Court had rejected his challenge to the constitutionality of aspects of the deal.
This morning, the Supreme Court ruled by a majority of four to three that the Constitution precludes the ratification of the deal as Irish law now stands.
The court also ruled by a majority of six to one that if certain amendments were made to legislation then the deal could be ratified without breaching the constitution.
However, the court said that was a matter for the Oireachtas.
The court ruled unanimously that the ratification of the deal was not an obligation necessitated by membership of the EU, and as such it could be constitutionally challenged.
It also found by a majority of four to three that ratification would breach the Constitution.
But it also held by the same majority that if there were certain amendments to the 2010 Arbitration Act then the deal could be ratified without being unconstitutional.
The Dublin South-Central TD had claimed the deal should be put to the people in a referendum and that its ratification might have an impact on the State's ability to introduce regulation, particularly in relation to the environment.
The decision was live-streamed on the YouTube channel of the Courts' Service.