Jeremy Corbyn quipped that fish recognised no borders during his fleeting visit on Friday.
The Labour leader also expressed concern about protecting environmental standards after Brexit while standing on a bridge over the River Foyle for photographs and selfies.
Hauliers sounded their horns and were rewarded with a wave as Mr Corbyn crossed the wide waterway which will form part of the UK’s border with the EU after Brexit.
Tensions have already been raised in the north west over competing territorial claims between Ireland and Britain to Lough Foyle’s oyster-rich waters, a number of miles north where the river widens to meet the Atlantic.
The Labour leader was accompanied by Ulster University Professor Deirdre Heenan as he walked the few metres across the busy span from Strabane in Co Tyrone in Northern Ireland into Lifford in Donegal in the Republic and posed for photos.
In 2015, Prof Heenan was part of a special independent commission set up by the Labour Party under Ed Miliband to make recommendations on tackling inter-generational poverty and social exclusion in Northern Ireland.
When the party lost the general election the plans were denied immediate implementation but during his visit Mr Corbyn voiced several times his desire to tackle the “desperate” poverty suffered by some in Londonderry.
He envisaged extra investment in infrastructure, and later he acknowledged that a general election may not be far away.
During a breakfast meeting with representatives from the Londonderry Chamber of Commerce, the Labour leader referenced the civil rights marches, 50 years ago this year, which descended into violence in a city which would be gripped by decades of bloodshed.
He was surrounded by ghosts of the past – in his City Hotel room in Derry there was a picture of SDLP founder and Nobel Laureate John Hume, he told the audience.
The veteran parliamentarian greeted nationalist “old friends” in the city who had turned up to see him, including the SDLP’s former Foyle MP Mark Durkan.
SDLP leader Colum Eastwood was also present as well as Sinn Fein MP Elisha McCallion.
Before a strongly anti-Brexit audience Mr Corbyn said he understood the anger which drove some in Labour constituencies to vote for separation and reiterated that he would not attempt to prevent it, rather to ameliorate or soften it.
He travelled with a retinue including veteran London MP Stephen Pound and shadow Northern Ireland secretary Tony Lloyd.
The leader promised to send his representative back to the city, which he said he had visited many times.
The concerns he heard focused on Brexit and how catastrophic a hard border would be for trade with a close neighbour.
Mr Corbyn was clear that he opposed such a development and was prepared to take action in Parliament.
Bonnie Anley, who is chairwoman of Foyle Port in the city, said: “Despite the impending negotiation deadlines, we, like many businesses here, feel there is still a lack of clarity in the key areas of trade, travel and customs.
“As a key driver of the north west economy we are keen that the post-Brexit arrangements are firmed up as soon as possible.”
Mr Corbyn later travelled to Harland and Wolff shipyard in East Belfast, which once built the doomed Titanic liner but now specialises in refitting cruise ships.