Sinn Féin leader Mary Lou McDonald has urged the Taoiseach not to accept any checks on the island of Ireland in the event of a no-deal Brexit.
Speaking in advance of a meeting of the party's Ard Comhairle in Dublin, Ms McDonald also said the party was now formally preparing for a Westminster election.
"We are very concerned to hear the Taoiseach think out loud and to concede the point that there might be checks anywhere on the island of Ireland," Ms McDonald said.
"There can't be any check anywhere on the island of Ireland. That's entirely unacceptable. It would be an act of absolute political vandalism and wherever these checks might be located it would represent a serious breach of the Good Friday Agreement," she said.
Asked how the European Single Market would be protected in the event of the UK crashing out of the EU, Ms McDonald said the Good Friday Agreement took precedence.
"There is an absolute obligation to protect our economy and to protect that market yes, but the greater obligation is to protect the Good Friday Agreement, to protect the peaceful dispensation built over decades.
"It shouldn't come down to a choice between the protection of the Single Market and protection of the Good Friday Agreement, the challenge is to achieve both," she said.
Ms McDonald also said the Government had to prepare for the ultimate removal of the border following a no-deal Brexit.
"Europe has stated categorically there is a way back into the European Union for the North in the event of re-unification," she said.
She added that if the Conservatives could not reconcile themselves to the Backstop and there is a no-deal Brexit, "we are straight into the terrain of preparation to remove the border.
"We cannot envisage a re-partition of Ireland almost a century on".