Taoiseach Leo Varadkar has said there can be no withdrawal treaty for Britain's departure from the European Union that does not contain a so-called "backstop" option on avoiding a hard border.
Arriving in Brussels for a two-day summit of EU leaders in Brussels, Mr Varadkar said he welcomed the fact that, as he put it, the British government had changed its position and had agreed that there does need to be a backstop within the divorce treaty.
He said: "I very much welcome the fact that a protocol that had been ruled out only a few weeks ago, that the British government has now changed their position, that they do accept that there does need to be a backstop in the Withdrawal Agreement."
Under a deal reached between the EU and the UK in December, Britain pledged that - in the absence of other solutions - Northern Ireland would remain aligned to the rules of the EU's customs union and single market in order to avoid a hard border and to protect the North-South dimension of the Good Friday Agreement.
When that solution, known as Option C, or the "backstop", was converted into a draft legal text in February it was vehemently rejected by British Prime Minister Theresa May.
Mrs May has subsequently accepted - in a letter to the European Council President Donald Tusk - that there would have to be language on a backstop within the withdrawal treaty.
As a result of the letter, the EU27 are expected to give the green light to a two-year transition period coming into effect at the end of March next year.
The Taoiseach said he welcomed the transition period as it would be good for farmers and businesses in that nothing would change until the beginning of 2021.
Taoiseach says Brexit withdrawal treaty will have to be agreed by October pic.twitter.com/a22EKDXijh— RTÉ News (@rtenews) March 22, 2018
He told reporters: "For the best part of two years after the UK leave the European Union they'll still be bound by rules and regulations on European laws that they won't have an input to."
The Taoiseach said Ireland was willing to explore other solutions.
The so-called Options A and B, which Britain prefers, would attempt to avoid a hard border through the future free trade agreement between the EU and UK, as well as the use of customs exemptions and technology.
"We've always said that we're willing to examine alternative solutions, particularly ones that bring the UK closer to the EU and may even mean we don't need a backstop at all, or don't mean we need one as explicit as we do now," he said.
He added: "Even though we very much welcome the transition, there cannot be a Withdrawal Agreement without a solution to the Irish border and a hard border being avoided.
"We'll be standing very firm on that."
Mr Varadkar said a legal text on the backstop would have to be agreed by October so that the treaty could be ratified in time by member states and the European Parliament.
He said: "We have absolute unconditional support from across Europe from all member states that there can be no agreement unless a hard border is avoided, and I haven't seen any departure from any European institution or the Task Force or any other member state on that absolute position."
Mr Varadkar held a bilateral meeting with British Prime Minister Theresa May this evening.
They spoke about Brexit, Northern Ireland, and Russia.
Mrs May reassured Mr Varadkar of her commitment to the December Brexit blueprint.
They both looked forward to the negotiations next week at official level in Brussels on the border and will discuss Northern Ireland again after Easter.
The Taoiseach also expressed solidarity and support following the chemical attack in Salisbury.
Earlier, as she arrived at the summit, Mrs May said she was hopeful of a "swift" move to talks on the UK's future trade relationship with the EU following the summit.
Leaders are expected to give their approval tomorrow to a draft agreement on the terms of the UK's withdrawal from the EU, including a post-Brexit transition period lasting until the end of 2020.
Mr Tusk has recommended endorsement of the deal, which he said would put off "the negative consequences of Brexit" for another 21 months after the formal date for the UK's withdrawal in March 2019.
Mrs May said: "I'm looking forward to talking about Brexit.
"We made considerable progress through the agreement on the implementation period, which will bring certainty to businesses and people.
"I look forward to the European Council endorsing that agreement and moving on swiftly to talk about the future partnership that we all want to build together."
Under the terms of the joint legal text agreed by UK Brexit Secretary David Davis, and the EU's chief negotiator Michel Barnier, the UK will be able to negotiate and ratify trade deals with outside countries during the transition period, to enter into effect on 1 January 2021.
Earlier this week, Mr Tusk expressed doubts over whether the remaining 27 EU states would give their approval to the draft transition deal.
Spain is understood to have insisted that it should retain a veto over any decisions on the future of Gibraltar.
But on the eve of the summit, Mr Tusk tweeted: "I have just recommended to EU27 leaders that we welcome, in principle, the agreement on transition.
"In practice, the transition phase will allow to delay all the negative consequences of Brexit by another 21 months."