Relations between the EU and the US are going through a "difficult period", according to the European Union's Ambassador to the US.

Speaking to RTÉ's Washington Correspondent about the impact of US President Donald Trump's administration, Irishman David O'Sulllivan said: "The disruptive policies practiced by this administration are inevitably going to disrupt."

But Mr O'Sullivan, who finishes up in his job next week, said the transatlantic relationship remained "absolutely crucial for Europe and for the United States" and that the fundamentals remained very strong.

He said: "President Trump was elected on a platform of disruption and challenging the status quo, so it was not entirely unexpected that Europe came in for a certain amount of criticism."

But he said the EU had engaged with the US President and that European Commission President Jean-Claude Juncker had agreed a common agenda on trade issues when he met President Trump last summer.

He said: "Yes, there are differences of opinion, but we're trying to find common ways forward."

Mr O’Sullivan described the downgrade last month of the diplomatic status of the EU Delegation in Washington by the Trump administration as an "unfortunate incident".

However, he said it was not really a downgrade of the office, but a change in diplomatic protocol.

He said it had caused disappointment, but he added "we understand the matter is under review" and he hoped there could be a return to previous practices.

Asked if relations had been damaged, he said he felt the relationship was very resilient and "it would take a lot to damage it".

In relation to Brexit, Mr O'Sullivan said there was a lot of interest in the US about the UK leaving the EU but he said many were perplexed by it.

He said: "People are now understanding the enormous complexity of the UK trying to extricate itself from 45 years of close association with European integration and the day-to-day consequences this will have across a wide range of issues."

Mr O'Sullivan said people were also perplexed by the disarray in Westminster at the moment. He said the main interest was in the EU and the UK finding a satisfactory agreement to permit an orderly Brexit, which he said would be in the best interest of US investors.

Asked about the potential impact on Ireland, he said US support for the peace process, and particularly the Good Friday Agreement, continued to resonate.

He said there was a huge support for the position of the Irish Government and the British government that there should be no return to a hard border.

He said the current Irish Government had handled the situation well and had done a great job at making Ireland's case.

"At its heart, the European Union is a project about peace", and he said the end of 30 years of the Troubles resonated strongly with our European partners.

As he reaches retirement, Mr O'Sullivan said one of the high points of his career had been celebrating EU enlargement on 1 May 2004.

He said he could not predict the outcome of the US elections in 2020, but said it would be a "tough political campaign".

On the topic of car tariffs, he said that European Commission President Jean-Claude Juncker had said today that President Trump had given his word that he would hold off on tariffs on EU cars.

Mr O'Sullivan said he expected this to be the case and that neither side should introduce new tariffs while talks were ongoing.

He added: "We didn't think this was justified for steel and aluminium - we think it is absolutely not justified to do that over the question of car tariffs."