Bill Clinton has warned that some Brexit voters may only now be realising the impact of their decision.
The former US president said the choice between community and tribe will determine how the world meets the threat posed by climate change, inequality and other great challenges.
After being honoured by Dublin City University for his work on peace and reconciliation in Northern Ireland, Mr Clinton said he used the example of the Good Friday Agreement shamelessly around the world.
But he warned that the Brexit vote was about people thinking differences are more important that what they have in common.
"Now, there are lots of people who think they are less human," he said.
"Now given the economic inequalities and the rapid pace of social change and all the upheaval that's going on .... people are reassessing whether what we have in common is more important than our differences.
"A lot of people begged to differ.
"That's really what the Brexit vote is all about."
Mr Clinton was awarded an honorary doctorate of philosophy, DCU's highest honour.
Alongside him were Sister Stanislaus Kennedy, a lifelong social justice campaigner who founded Focus Ireland, the country's biggest homeless charity, and the Immigrant Council of Ireland.
Also honoured at the event in the Helix was Martin Naughton, founder of manufacturing giant Glen Dimplex.
Mr Clinton reflected on his own legacy with the Good Friday Agreement, signed in Belfast in 1998, mid-way through his two terms in the White House.
Ultimately the accord was brokered after he took a more hands-on approach to US diplomacy on the island of Ireland at the height of the Troubles.
Mr Clinton travelled direct to Northern Ireland for private meetings with politicians after the ceremony - his intervention in the political stalemate delayed by a day because of Storm Ophelia.
In a lengthy address to hundreds of invited guests and students in the Helix in DCU, Mr Clinton warned about inequalities and divisions.
"The world is now in a conflict whether we should stop our mingling with others at the tribal level or whether communities are better; whether diverse groups make better decisions and create wealth and life and opportunity or homogenous ones do as they don't push us so hard and we feel more secure," he said.
"We can't get away from each other so we should look at our neighbours without regard to their race, religion, their orientation or whatever."
Mr Clinton raised concerns about the rise of nationalist parties in Germany, France, the Netherlands and Austria and also the impact of Brexit.
Mr Clinton added: "All partnerships that are community-based are held together not because everybody agrees with everybody else, not because we don't still have our particular identities, but because co-operation is better than conflict or isolation in any environment in which you must be in touch with others.
"It's a simple proposition. But we are re-litigating it now."
Mimicking a voter in the referendum on Britain's place in the EU, he said: "I'm sorry we can't stay together, we had a disagreement. Oh my God, I didn't know I was going to lose that customs thing and all these economic benefits. Why didn't anyone tell me that?"
Mr Clinton described the Good Friday Agreement as a "wonderful blinding moment of bigness".
"The children of God and humans chose community," he said.
"Nobody abandoned their tribe, they just lived in the same neighbourhood.
"We must make that choice again. How we think will determine what we do with every other challenge facing us. It is the most important thing."
Mr Clinton called for universities to be places of honest discussion about whether it is better to live in a tribe apart, or a community of many tribes with shared values and mutual respect.
"Believe me, it is just one example of what is dominating the discussion all over the world," he said.
"Some thought needs to be given before we abandon community to return to tribe.
"You can keep your tribe."
He added: "You don't have to give up your tribal identity to respect your larger humanity, without which the world will not continue to thrive.
"How we think about this will determine how we deal with everything else, with climate change, with the largest species destruction rates in 10,000 years, with vast inequalities."
In a citation for Mr Clinton, Professor Gary Murphy, of DCU's School of Law and Government, said: "Peace-making is always difficult, but there can be little doubt that the conflict in Northern Ireland was ultimately resolved because that great beacon of liberty, the USA, decided that it could use its influence to make the vital difference.
"That fateful decision was taken in the Oval Office by President Bill Clinton."
Prof Murphy also paid tribute to Mr Clinton's work on the Balkan wars, responding to the 2004 tsunami and 2010 earthquake in Haiti.