December's deal to avoid a hard Border still stands and the preferred option to secure that is through a comprehensive free-trade deal, the Taoiseach has said.
But Leo Varadkar said the "tricky" part of the December deal is "and always was going to be" delivering on the commitment to avoid a hard Border.
It comes just days after the EU's chief Brexit negotiator reiterated that a UK exit from the single market and customs union would leave border checks unavoidable.
Mr Varadkar said the Irish and UK governments have now agreed to co-operate at official level to work out how the commitments given in December to avoid a hard Border can be translated into reality.
"Both the British government and the Irish Government are very much of the view that the agreement that was made back in December stands, and that we both prefer option A as the best option by which we can avoid a hard Border in Ireland, and that is through a comprehensive free-trade and customs agreement involving Britain and Ireland," Mr Varadkar said.
"That is the best way we can avoid any new barriers north and south, and east and west, and we have agreed to work together at official level to see if we can explore solutions as to how that can be achieved over the coming weeks and months."
Mr Varadkar was speaking at Stormont after meeting with UK Prime Minister Theresa May. Both leaders were in Belfast amid expectations that a deal to restore power-sharing in Northern Ireland is close.
The Taoiseach said December's deal ensures the protection of the common travel area and the protection of joint citizens' rights between Ireland and Britain, and the protection of peace funding.
"All that is absolutely bulletproof. The tricky part is, and always was going to be, how we deliver on the commitment to avoid a hard Border," he said.
The objective of December's deal is to reach a new UK-EU relationship - but if this doesn't occur, then the UK will come up with specific solutions.
A final element was added at the request of the Irish Government, which essentially says if no final deal can be done, the UK will maintain "full alignment" with the EU rules required to ensure that north-south co-operation in terms of trade, regulations and standards, can continue.
Mr Varadkar said it was the view of both the British and Irish Governments that the preferred option was not the final element.
Meanwhile, Mrs May urged the DUP and Sinn Féin to make a final push to strike a deal to restore power-sharing.
She said there was a basis for an agreement, and met both parties. The Taoiseach also held party meetings, but did not meet the DUP.
Tensions between the DUP and Government have been strained by Brexit. But Foreign Affairs Minister Simon Coveney said people "shouldn't read too much into" the fact the Government did not meet the DUP.
The North has been without a power-sharing administration for more than a year, with Sinn Féin's demand for an Irish Language Act the main stumbling block. It has been speculated a deal is now on the cards. Mrs May said the DUP and Sinn Féin have been working hard to close the remaining gaps.
"Some differences remain, but I think there is the basis of an agreement here, and I have been urging the parties to make one final push for the people of Northern Ireland," she said.
"We believe we are close to an agreement which, certainly, we can put to our grassroots and to the community as a whole," said Sinn Féin President Mary Lou McDonald. "We are not exactly there just yet...but there is nothing insurmountable if there is the political will to reach an agreement."
DUP leader Arlene Foster said good progress has been made.