Last summer Brendan O’Leary, a professor at the University of Pennsylvania, was in Ireland researching and writing his forthcoming book, tentatively titled Understanding Northern Ireland: Passages from Colonialism to Consociation and Confederation, 1603-2017, when the BREXIT referendum television broadcast interrupted his work.

The June 23 referendum termed BREXIT, an amalgamation of the words “British” and “Exit,” resulted in the United Kingdom voting to leave the European Union, a partnership of 28 nations designed to expand economic growth after World War II. While the referendum passed in England and Wales, voters in Scotland, Northern Ireland and Gibraltar wanted to remain.

“I’d hoped that the referendum would go the other way,” says O’Leary, a dual American and Irish citizen. “It would have been a nice finishing point to the book. Instead, we have a revolt against reason that could affect the stability of the peace process that I played a minor role in helping to negotiate.”

O’Leary, a specialist in power sharing and constitutional reconstruction, was influential in drafting the Good Friday Agreement of 1998, offering a peaceful resolution to nearly 30 years of conflict in and across Northern Ireland.

He says technically BREXIT should be referred to as “UKEXIT” as “it is not right to use ‘Britain’ to refer to the whole of the U.K.”

O’Leary says that while it is possible to reach constitutional compromises, UKEXIT will result in far-reaching impacts on trade, borders, citizenship laws, governmental power and, above all, Northern Ireland’s peace agreement.

“The consensual solution would be to negotiate the secession of England and Wales from the E.U. but to allow Scotland and Northern Ireland to remain,” says O’Leary, who wrote a blueprint for this in the Dalriada Document, named after an ancient polity that linked Scotland and Northern Ireland. It was published in the Political Quarterly.

UKEXIT raises issues of secession between Scotland and the U.K. as well. In 2014, Scots voted no to a referendum on independence from the United Kingdom. However, none of the parliamentary districts in Scotland voted to leave the E.U. in the 2016 vote.

“In the old English view, the Scots have no option but to exit against their will and without the consent of their own parliament,” O’Leary says. “If Scotland and Northern Ireland leave against their wishes, I think it will eventually lead to the end of the U.K.”

He notes that UKEXIT contributed to sparking fresh elections in Northern Ireland.

“These elections signify renewed instability there, partly triggered by the E.U. question,” says O’Leary. “Many in Northern Ireland fear that a U.K.-wide exit would restore a hard border across Ireland and strip away components of the 1998 Good Friday Agreement.”

O’Leary says it’s pretty clear that the U.K. conservative government is intent on leaving the E.U. The consequences, however, will not only erect a border between Ireland and the U.K., but also lead to a re-evaluation of citizenship and migration laws.

“If the border rests in the Irish Sea, it would be minimally disruptive,” O’Leary says. “But, if a hard border is restored across the land-mass of Ireland, which had been demolished by the U.K. and Ireland’s joint membership of the E.U., then there may be trouble ahead.”

A constitutional compromise would alleviate uncertainty in Northern Ireland, while easing the pressure from Spain for the U.K. to surrender its power over Gibraltar. Relocating enterprises to E.U. zones within the U.K. would soften the negative economic impact of an entire U.K. exit, O’Leary adds. It would also allow England and Wales to experiment with their own preferred policy freedoms.

Soon after he saw the referendum on TV, O’Leary, an internationally recognized political scientist and member of the Irish Foreign Policy network who is regularly in contact with the Irish Department of Foreign Affairs, received an avalanche of requests for commentary.

He has shared his UKEXIT proposal ideas with larger audiences, including Scottish government officials, thanks to “a combination of old-fashioned networks, the prestige of being a researcher at an Ivy League university and modern media,” he says.

He has also published his ideas in the London School of Economics blog and participated in talks on legal strategies at Queens University in Belfast.

“The British government,” says O’Leary, “will have to consider the consequences of imposing the departure of all of the U.K. against the majority will in Scotland and Northern Ireland.”

He recommends the U.K.’s Supreme Court base its decision on contemporary thinking.

“A wise Court,” O’Leary says, “would have favored the new constitutional pluralism, fully preserved the Good Friday Agreement and respected the Scottish Parliament’s right to be consulted about changes that affect its powers.”

Amendments to the U.K.’s planned exit are now being considered in the House of Lords. If these are rejected, says O’Leary then negotiations for the U.K. to leave the E.U. may begin in late March. If there is a delay, it could be another six months.

“How negotiations will play out and what those repercussions will be no one knows,” O’Leary says, stating that the E.U. will present the U.K. with a bill nearing 60 billion euros and will refuse to open negotiations on trade until the U.K. agrees to fork over the money.

“The British position will be one of intermittent realism and a posture of incredible fantasy. They’ll want to have parallel negotiations on the cost of exit and the cost of trade, but the E.U. wants to be clear that the U.K. will accept that it has to pay the bill upfront,” O’Leary says. “We’ll see who blinks first.”