Senator Lynn Boylan has launched a High Court action against the Irish Government's implementation of the EU-Canada Comprehensive Economic and Trade Agreement.
Former MEP Lynn Boylan, Sinn Féin's Dublin Bay South by-election candidate, lodged papers this week, taking action against the controversial trade agreement.
Green Party TD Patrick Costello is also taking a case against the trade deal.
Climate campaigners say Ceta would hamper efforts to tackle the climate emergency because a mechanism within the deal, the Investor Court System (ICS) allows for multinational companies to sue a state for damages if it introduces new laws or policies that the company thinks will reduce its future profits.
Ms Boylan told thethat the move is not linked to the by-election and is not done in the name of Sinn Féin but by her as an individual.
"So it's a special court under Ceta for multinational corporations, while I believe Ireland has successfully traded without a private court since the inception of the State.
"I believe its constitutionality needs to be tested, whether it's constitutional to have a private court."
Although similar to Mr Costello's action against the State, the legal argument is technically different.
Ms Boylan's argument focuses on the committee that would be set up as part of the investor court, which Ms Boylan says can amend the rules or access to the court without Oireachtas oversight.