Mary Lou McDonald says she wants a “government of the left” after the next general election.
Several recent polls have shown strong gains for Sinn Féin, with the party now enjoying the highest level of support of any party in the State.
Deputy McDonald has failed to rule out working with either Fianna Fáil or Fine Gael.
However, she says she would rather form a government without those two parties.
She said: "Our commitment is to lead a government of change.
"The best government that can emerge from the next election is one without Fianna Fáil or Fine Gael.
"The worst outcome - as I predicted last time - is a government of Fianna Fáil / Fine Gael together."
Coveney confidence vote
Meanwhile, the Sinn Féin leader is also defending her party's decision to move a motion of no confidence in Foreign Affairs Minister Simon Coveney.
She has denied the move was a stunt, saying cronyism in Government needs to be tackled.
Minister Coveney is under pressure after his failed attempt to appoint his former Cabinet colleague Katherine Zappone as a UN Special Envoy.
Deputy McDonald said the Dáil should vote the Fine Gael Minister out of office.
She said: "Their culture of cronyism, favours for friends and insiders has come to the surface again.
"Minister Simon Coveney sought to make up a job for a friend and former colleagues.
"He kept it quiet and away from the Taoiseach for months - when he was caught red-handed, he went about covering his tracks."
TDs will vote on the measure this week, after the Dáil resumes tomorrow following the summer break.
Earlier, Tánaiste Leo Varadkar said he believes Minister Coveney will survive this week's vote - suggesting it would be disproportionate for his party colleague to lose his job over the controversy.