The President of the European Council, Donald Tusk, has told the Taoiseach, Leo Varadkar, that Ireland's interests come first in the Brexit negotiations.
Speaking in Dublin, Mr Tusk said nothing must be done to risk the peace process or the Good Friday Agreement.
He said every EU leader he has met has expressed support for Ireland's position.
He called on the British government to produce a "realistic solution" to avoid a hard border on the island of Ireland.
Mr Tusk said: "If in London someone assumes that the negotiations will deal with other issues first before moving to the Irish issue, my response would be 'Ireland First'."
"Peace process a priority"
Responding to Theresa May's criticism of the draft Withdrawal Agreement which the Commission published on 28 February, Mr Tusk said: "We also have to be clear that any backsliding on the commitment made so far would create the risk to further progress in Brexit negotiations."
"Since my last visit to Dublin I have spoken to virtually every EU leader and everyone without exception declared... that among their priorities are protecting the peace process and avoiding a hard border. The EU stands by Ireland."
Mr Tusk began his comments with some light-hearted weather references saying: "I may be from the east but I am not a beast".
But the tone quickly became more serious as he reaffirmed the EU position on the negotiations: "We also expect the UK to provide a specific and realistic solution to avoid a hard border. As long as the UK doesn't present such a solution, it is very difficult to imagine substantial progress in Brexit negotiations."
Mr Varadkar told the news conference: "I've always said that my preference is to avoid a hard border through a wider future relationship between the United Kingdom of Great Britain and Northern Ireland and the European Union.
"We're committed to playing our part in exploring this option, or alternative specific solutions, in a way that respects the structure of these negotiations and that will of course require further detailed progress to be put forward by the UK government.
"However we must have certainty that if a better option proves unachievable, the backstop of maintaining full alignment of Northern Ireland with those rules of the single market and customs union that apply in order to protect north-south co-operation and avoid a hard border."
Mr Tusk also referred to a recent speech by Chancellor of the Exchequer, Philip Hammond in which he called for financial services to be included in a future trade deal.
The EC president said: "Services are about common rules, common supervision and common enforcement, to ensure a level playing field, to ensure the integrity of the single market and ultimately also to ensure financial stability.
"Life different after Brexit"
"This is why we cannot offer the same in services as we can offer in goods. It's also why FTAs (free trade agreements) don't have detailed rules for financial services.
"We should all be clear that, also when it comes to financial services, life will be different after Brexit."
Responding to Mr Hammond's assertion that it was in the interest of both Britain and EU to ensure that UK-based financial services companies had easy access to the single market, Mr Tusk said: "I fully respect the Chancellor's competence in defining what is in the UK's interest. He must allow us to define what is in the EU's interest."
Democratic Unionist MEP Diane Dodds said Mr Tusk's comments suggest the EU will again hold up the trade talks if the December fall-back option of alignment is not legally translated.
"Far from putting 'Ireland first' this will generate further frustration among Republic of Ireland businesses dependent on access to their primary marketplace in the UK," she said.
"These include Irish agri-food firms which export around 40% of their produce to Great Britain and Northern Ireland."
SDLP leader Colum Eastwood said Mr Tusk's comments showed that "the EU is not messing about".
"It is crystal clear that the EU's commitment to protecting the interests of the island of Ireland is immovable," he said.