Addressing both houses of the Irish parliament, Michel Barnier said: “I am fully aware that some member states will be more affected than others.
“I want to reassure the Irish people: in this negotiation Ireland’s interest will be the Union’s interest … Brexit changes the external borders of the EU.
“I will work with you to avoid a hard border.”
In addressing both houses of parliament, Mr Barnier was given a privilege normally only afforded to visiting heads of state and prime ministers, joining luminaries like Nelson Mandela and Bill Clinton.
Mr Barnier said there was no reason why the EU cannot have a “strong relationship” with the UK after it leaves, but Brexit will inevitably have consequences.
Protecting Ireland’s interests as part of the EU would be an important part of the exit talks, with efforts to protect the peace process forming a key element alongside avoiding a hard border.
Echoing comments made last week, Mr Barnier said Brexit would “come at a cost” to both the UK and the remaining 27 members of the EU, but his objective was to reach a “fair deal” with London.
He called for “mutual respect” in negotiations, in an apparent reference to the diplomatic row caused by leaks following European Commission president Jean-Claude Juncker’s Downing Street meeting with Theresa May.
The Prime Minister has accused EU politicians and officials of trying to influence the outcome of the 8 June election.
Mr Barnier said: “If we put things in the right order, if we negotiate with mutual respect, without any kind of aggressivity … if we are open to finding solutions, there is no reason why a strong Europe cannot maintain a strong relationship with the UK.”
Recognising that Brexit poses specific issues to Ireland, he said: “I want to reassure the Irish people: in this negotiation Ireland’s interest will be the union’s interest.
“We are in this negotiation together and a united EU will be here for you.”
He repeated the EU’s position that the issue of the UK-Ireland border, citizens’ rights and “the financial settlement” must be the first priorities for talks.
“We first must make sufficient progress on these points before we start discussing the future of our relationship with the UK.
“The sooner this will happen, the better.
“If the conditions are right, a close partnership with the UK is in everybody’s interest. And in Ireland’s interest in particular.”
Mr Barnier said he wanted the relationship between the UK and EU to extend beyond trade, with co-operation on other cross-border issues.