The Taoiseach is among 27 leaders in Brussels for a two-day summit
The Taoiseach Leo Varadkar says Ireland will have to remain vigilant in phase two of talks on Brexit.
This is despite getting what he says is a strong agreement that there will be no hard border.
Mr Varadkar is among 27 European Union leaders who are in Brussels for a two-day European Council summit.
Despite assurances by MEPs that there will be no back-tracking on the agreement, some are still concerned the deal is not legally binding.
Mr Varadkar says the deal provides guarantees if no agreement can be reached on trade with Britain in future - known as a 'backstop' scenario.
"We were very happy with what was agreed last week, which is the joint report, and that's very much a political agreement.
"What has to happen now over the next few months is turning the commitments and principles in the joint report into the withdrawal agreement - which is a legally binding international agreement which allows the United Kingdom to leave the EU."
"What we're saying here is that within this backstop scenario, the United Kingdom and Northern Ireland in particular, would maintain full alignment with the rules and regulations of the internal market and the customs union as a backstop arrangement - and that gives us a very strong assurance that there won't be a hard border on the island of Ireland.
"But we're not complacent, either - this is politics, we're going to need to stay very engaged in the months and years ahead - and very vigilant, too".
While British Prime Minister Theresa May has insisted she is "on course to deliver Brexit" as she tried to shrug off a House of Commons defeat on key legislation.
Speaking as she arrived at the summit, Mrs May attempted to play down the significance of a government loss on the UK's EU Withdrawal Bill on Wednesday night.
A group of 11 Tory rebels forced an amendment to the flagship bill to demand a "meaningful vote" on any Brexit deal, despite Mrs May having warned it could put at risk an "orderly and smooth exit from the EU".
She admitted she was "disappointed" with the defeat, but suggested it was not a setback.
She told reporters: "I'm disappointed with the amendment but actually the EU Withdrawal Bill is making good progress in the House of Commons and we're on course to deliver on Brexit."
Mrs May added: "We've actually had 36 votes on the EU Withdrawal Bill and we've won 35 of those votes with an average majority of 22.
"So the bill is making good progress, we're on course to deliver Brexit and we're on course to deliver on the vote of the British people."
On the loss at Westminster, Mr Varadkar said: "I've absolute faith and confidence in (Theresa May) that she speaks for the Government of the UK".
At the summit, leaders will also address defence matters.
They are expected to welcome the launch of the Permanent Structured Cooperation (PESCO) and to review progress on EU-NATO cooperation.
Additional reporting: IRN