European Union chief negotiator Michel Barnier walked the historic walls of Londonderry and was shown the Bogside area of the city where British soldiers opened fire on Bloody Sunday.
Mr Barnier was given a whirlwind tour after a meeting with businesses and cross-border groups in the Guildhall.
A guide walked with Mr Barnier and gave him a brief history of the city, pointing out the Bogside from the top of the city walls.
Thirteen people died when members of the Parachute Regiment opened fire on civil rights demonstrators in 1972. A 14th person died later.
All those killed on Bloody Sunday were later pronounced innocent by a public inquiry led by Lord Saville, and then-prime minister David Cameron said the killings were unjustified and unjustifiable.
Dr O’Neill said Mr Barnier was very “forthright”, adding that they had a “very frank open discussion”, which he said was very useful to everyone in the room.
He said implications for research funding was discussed in the meeting, which lasted about one hour.
“I think he’s minded to come up with a solution that works best for the unique scenario that we have in this region.
“We’re as positive as we can be in, I suppose, what’s an overall negative situation,” he said.
Mr Barnier said he was delighted to be in the “historic and vibrant” city of Derry, and said he had already enjoyed a walk on the Peace Bridge.