An all-Ireland forum on Irish unity should be convened urgently, Mary Lou McDonald has said.
The Sinn Fein president claimed that momentum is building around the issue and arrangements are needed to manage that.
On Friday Taoiseach Leo Varadkar said moderate unionists and nationalists could be spurred to consider a united Ireland if there is a no-deal Brexit, but it would be provocative for his government to begin making preparations now.
Ms McDonald said: "The Taoiseach must make clear that in the event of no-deal Brexit that Irish unity is the solution to averting a hard border on our island.
"A referendum on unity is expressly provided for in the Good Friday Agreement, an agreement that was endorsed by the people of this island, north and south, overwhelmingly over 20 years ago. There is nothing provocative about wanting a united Ireland and the Government has a responsibility to lead from the front."
Mr Varadkar had said those uncomfortable with a "nationalistic" Britain could join forces to support Irish unity and continued membership of the EU.
He said it would be provocative for the Irish Government to take steps towards engineering a united Ireland now and the country simply wants to maintain the status quo.
Ms McDonald said: "There is a momentum building for Irish unity and we need political, institutional and legislative arrangements in place to manage that.
"That preparation must include the convening of an all-Ireland forum on Irish unity without delay."