The Taoiseach Leo Varadkar and European Commission President Jean-Claude Juncker have dashed any hope of changes to Britain's EU withdrawal agreement.
The two men spoke by phone on Wednesday evening.
They both agreed that the Brexit withdrawal agreement is "a balanced compromise and the best outcome available".
While they also agreed to work to provide reassurance to the UK, they reiterated that the deal "cannot be re-opened or contradicted".
It comes after British Prime Minister Theresa May held talks with the German Chancellor Angela Merkel and Dutch Prime Minister Mark Rutte on Tuesday to discuss concerns over the agreement.
She was meeting EU leaders in an attempt to "seek reassurances" on the Irish border backstop.
Speaking in Brussels on Tuesday evening, Mrs May said: "The backstop is a necessary guarantee for the people of Northern Ireland - and whatever outcome you want, whatever relationship you want with Europe in the future, there's no deal available that doesn't have the backstop within it.
"But we don't want the backstop to be used, and if it is we want to be certain that it is only temporary.
"And it's those assurances that I will be seeking from fellow leaders over the coming days."
A planned House of Commons vote on the agreement for was scrapped on Monday, after Mrs May admitted it would not be passed by MPs there.
Mr Varadkar and Mr Juncker will meet at the European Council meeting on Thursday.
