The Fianna Fáil leader has called on Sinn Féin to state clearly if it is or is not in favour of EU membership.
Micheál Martin says the party in the North wants the UK to remain in the EU.
However he says it wants an in-out referendum in the Republic when the Transatlantic Trade and Investment Partnership is agreed between the US and EU.
Speaking in Wexford, Micheál Martin said, as usual, it shows up the double standards of Sinn Féin.
"If the TTIP is passed without a referendum, then they're recommending a referendum in Ireland in relation to the continued membership of Ireland in the European Union.
"Now this is something that's gone below the radar in the discussion about Brexit."