Green Party TD Patrick Costello has lodged papers in the High Court regarding the EU-Canadian trade deal, known as CETA, as he believes a referendum may be required for its ratification.
The Dublin South Central TD said his legal advice is that the Comprehensive Economic Trade Agreement involves the transfer of sovereignty and judicial power which is incompatible with the Constitution.
Mr Costello said he was concerned that if an element of the deal known as the Investor Court System came into force, without a referendum, it would be contrary to Article 15 and Article 34 of the Constitution.
CETA has already come into operation but has yet to be ratified by the Dáil.
Mr Costello and his Green Party TD colleague Neasa Hourigan have expressed opposition to the deal, which has now been referred for consideration to Oireachtas Committees.
Opposition parties, like Sinn Féin, have argued that CETA should be opposed.
In a statement, Mr Costello said: "It is of fundamental importance that the members of the Dáil, myself included, who would vote on whether to ratify CETA know that our votes are constitutional. Only the courts can give us complete certainty on that question. "
He clarified: "I have taken this action as an individual citizen and it is a personal action."
Mr Costello also explained his opposition to Investor Courts saying: "... as seen in other jurisdictions they are hugely problematic since they allow for the governments to be directly sued by companies outside of existing legal systems. This in turn may affect how policy makers decide policy in the long run.
"As such, it is of crucial importance that if CETA is to be ratified, then the process used is the correct one."
Tánaiste Leo Varadkar has suggested that Mr Costello taking a High Court challenge against the Government is worse than voting against it in the Dail.
He was responding to comments at a meeting of the Fine Gael parliamentary party this evening about Mr Costello's legal action.
Mr Varadkar told the meeting of Fine Gael TDs, Senators and MEPs that he was unsure whether Mr Costello was taking the case against the Government or him as Minister for Trade.
He said he did not know which was worse - suing the Government of which you are part, or voting against it in the Dáil.