Enda Kenny is confident the European Union will not allow "one of the most divisive borders in the world" to be reimposed on the island of Ireland after Brexit.
In what has been billed as a major policy speech, the Taoiseach said Ireland has "no choice" but to successfully resist a renewed land frontier once the UK leaves the EU.
"I have stressed this point to every European leader I have met," he said.
"In addition, all Government ministers have engaged intensively with their counterparts to ensure that Ireland's position is well understood."
Mr Kenny added: "I am confident that the European Union will not bring us back to a border of division."
Ireland will be negotiating from a position of strength as one of the 27 EU countries sitting across the table from Britain after Theresa May triggers Article 50, which starts the process of the UK pulling out of the EU, the Taoiseach said.
But he warned a political vacuum in Northern Ireland, which is facing a snap election after the collapse of the power sharing executive, could relegate the region's best interests during the Brexit talks.
In his keynote address to an Institute of International and European Affairs gathering, he said he would "do my best to put forward the interests of the North".
But he warned all political forces on the island of Ireland needed to come together if there was to be any chance of success.
"We have no choice but to work together, North and South, all of us," he said.
Mr Kenny also defended what he characterised as his minority Fine Gael-led government's focus on Northern Ireland in the lead-up to the imminent Brexit negotiations.
"Fewer than 350,000 people voted for Brexit in Northern Ireland, out of a total population of over 1.8 million," he told the audience at Dublin's Mansion House.
"But every man, woman and child in Northern Ireland will be affected by the outcome.
"And the vast majority of those affected are entitled to be Irish, and therefore EU, citizens.
"When I hear people say the Government is too focused on the North, I urge them to consider how you would feel if Ireland was to be removed from the European Union in similar circumstances."
Mr Kenny said Brexit had led Ireland to a crossroads facing unprecedented political, economic and diplomatic challenges as well as "challenges to our peace, and challenges to our prosperity".
"How we deal with it in the months and years ahead will define the future of our island for decades to come," he added.
"We must make sure that we shape that future for ourselves."
Mr Kenny said he utterly rejected any suggestion Ireland will follow Britain out of the EU, adding he still believed Brexit will be bad for the UK.
The mood of unity in defence of European values and ideals was palpable at the recent Summit in Malta and he believed that mood of unity would guide Ireland in the time ahead, he said.
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