Mary Lou McDonald says any talk of customs checks on or near the Irish border is "nothing short of political vandalism".
The Sinn Féin president and the party's deputy leader Michelle O'Neill are in Dublin today for an Ard Comhairle meeting on the latest developments on Brexit.
Earlier this week, the Taoiseach Leo Varadkar said that some checks on goods and animals may need to take place near the Irish border in the event of a no-deal Brexit.
He stressed that avoiding the return of a hard border is a priority for the Government "in all circumstances".
Deputy McDonald said a hardening of the border is completely unacceptable and the Taoiseach's recent border comments are 'extremely unhelpful'.
She argued: "I think any talk of checks on the island or any talk of a hardening of the border on the island of Ireland is nothing short of political vandalism.
"I think the Taoiseach needs to stand up and be counted - now is not the moment to soften our line or to lose our concentration."
She added: "We have all agreed, right across Irish life and politics, that there can be no damage to the Good Friday Agreement... that there can be no damage to our all-Ireland economy... that there can be no hardening of the border.
"Now we need to stand firm on that ground - that is the duty and the job of An Taoiseach in his dealings with all of our international allies, European and otherwise, and certainly in his dealings with Boris Johnson and the British state."