The EU's chief Brexit negotiator has emphasised there will have to be some form of customs controls between NI and the Republic after Brexit.
Michel Barnier was addressing at a joint session of the Irish houses of parliament in Dublin.
He is the first non-head of state or prime minister to make such an address.
Mr Barnier told Irish parliamentarians he had a duty to speak the truth and that the "UK's departure from the EU would have consequences".
He said the Irish border issue would be one of his three priorities in negotiations.
But he emphasised that customs controls were part of EU border management.
The British and Irish governments have both said they do not want a return to customs posts on the border.
The EU's negotiating guidelines call for a "flexible and creative" approach to the customs issue.
They also state that issues relating to the Irish border will have be resolved in the first phase of its talks with the UK.
No solid plans have yet been advanced by either the EU or the UK.
Mr Barnier said that whatever happened in negotiations, "nothing should put peace at risk".
He added that he wanted to "reassure the Irish people" that in the Brexit negotiations the Republic of Ireland's interest will be the EU's interest.
Mr Barnier said the EU wants the negotiations with the UK to succeed.
"We will need to negotiate a 'bold and ambitious', but fair, free-trade agreement," he said.
Addressing Mr Barnier in the joint committee, Sinn Féin President Gerry Adams told him he wanted to see a border poll within the next five years.
The taoiseach (Irish prime minister) has described Brexit as being bad bad for the UK, for Europe and the Republic of Ireland.
Enda Kenny said it challenges Ireland's peace and prosperity, although he said the country would maintain its close relationship with the UK.
The Republic of Ireland's economy is particularly vulnerable to any new tariff or regulatory barriers with the UK, which may arise as a result of Brexit.
The chief economist of the Irish Central Bank has warned that within 10 years of a "hard Brexit", the number of people employed would be 40,000 fewer, compared with a no-Brexit scenario.
Gabriel Fagan said that some small and medium-sized Irish businesses are "likely to be among the hardest hit by Brexit".
Seán Ó Fearghaíl, the speaker of the Dáil (Irish parliament), said Mr Barnier's address was "timely and appropriate" given that Brexit "could have a greater impact on Ireland than any on other EU state".
"As legislators there is an onus on us to be fully informed on the implications of Brexit on this country in particular and on the EU in general and to communicate our particular concerns on Brexit," he added.
Mr Barnier is expected to visit a food production business close to the Irish border on Friday.