EU border management means custom controls, warns Michel Barnier
Customs controls are part of the EU's border management, Michel Barnier warned people in Ireland.
The measures protected the single market, food safety and standards, the EU's chief Brexit negotiator added.
But he promised to help the Republic avoid a hard border with Northern Ireland after Brexit and recognised the unique position facing it with its shared frontier and strong economic ties to the UK.
He declared: "We have a duty to speak the truth. The UK's departure from the EU will have consequences.
"Customs controls are part of EU border management. They protect the single market. They protect our food safety and our standards."
Businesses and politicians in the Republic and Northern Ireland want to avoid the restrictions on movement of the past.
Brexit Secretary David Davis has said the UK would adopt technology to cover the transport of goods between Northern Ireland and the Republic.
The British Government wants a "frictionless" arrangement, to avoid a return to the borders of the past when the area was heavily militarised because of the IRA threat.
Mr Barnier said Ireland's interest would be shared by the entire EU.
"European integration helped to remove borders that once existed on maps and in minds. Brexit changes the external borders of the EU.
"I will work with you to avoid a hard border."
The UK's only land frontier with the EU is in Northern Ireland. It is invisible, with goods and people passing freely, its security apparatus from the 30-year conflict gone.
Mr Barnier has pledged to make it one of his three priorities for the first phase of the negotiations and is due to visit it on Friday.
He told Irish parliamentarians in Dublin: " Because of its historical and geographical ties with the UK, because of your shared border and strong economic links, Ireland is in a unique position."
With the depreciation of sterling, Brexit was already having an impact on the value of Irish exports to the UK, in particular the agri-food sector, the senior negotiator added.
He said: "And many in Ireland fear the return of tensions in the North.
"Today, in front of these two houses, 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."
On Friday he will travel to the border with Northern Ireland and meet farmers and workers in a dairy co-operative.
He added: "In Northern Ireland, lifting the borders took time. Only 15 years ago did check-points and controls totally disappear.
"Thanks to the Good Friday Agreement that ended decades of violence."
He said he understood the EU's role in strengthening dialogue in Northern Ireland and supporting the 1998 Agreement which largely ended the violence.
Later Mr Barnier met Taoiseach Enda Kenny at his south Dublin offices and signed the visitors' book.
He said: "I am very aware of the concerns on the part of the Irish people and I am ready to find solutions, respecting the market rules but also taking into account the unique situation of Ireland."
He said he hoped next month "as soon as possible" to find agreement on key points where the decision to leave the EU had created "huge and serious" uncertainty.
"I am ready to work and I am ready to work in total confidence and trust with the Irish Government."
Mr Kenny said Ireland would have "open access" to Mr Barnier and his team as the negotiations commence.
Sinn Fein president Gerry Adams said the economic implications of customs posts for the two economies on the island and especially for the border communities were enormous.
"Currently, island-wide trade generates over three billion euros annually.
"Around 60% of the North's exports to the EU are to the Irish state.
"Over 30,000 people regularly commute across the border between the North and South for work or study.
"Brexit puts all of this at risk and with it the jobs of those exporters who may now face customs posts."