Former Taoiseach John Bruton has warned of the risk of clashes on the high seas in the wake of Brexit.
Mr Bruton, who previously served as the European Union's ambassador in Washington, said it is easy to imagine physical confrontations over fishing rights in the seas around Britain and Ireland.
And he urged Northern Ireland's Democratic Unionists to get the Customs Union and jurisdiction of the European Court of Justice onto the Brexit negotiating agenda.
Mr Bruton said fishing rights are a highly emotional and symbolic issue.
"Fish do not respect territorial waters. While fishing boats can, in theory, be restricted to territorial waters, fish cannot," he said.
"Overfishing in one jurisdiction affects the livelihood of fishermen in another. Conservation is vital. Who will adjudicate on this, 10 years from now? Will there be quotas? Who will allocate them?
"In the absence of agreement, one can easily envisage clashes, even physical clashes, in seas around us."
Mr Bruton issued the warning as he delivered the Grattan Lecture in the Irish Embassy in London.
He called on the DUP - on course to prop up Theresa May as Prime Minister - to use their influence to reopen questions relating to customs and human rights.
"I hope that these are thoroughly looked at again, in an open-minded way, in the inter-party negotiations and the options properly debated. That debate did not take place in the general election campaign at all," Mr Bruton said.
The DUP are said to favour a soft Brexit with no physical border controls on the island of Ireland in order to maintain the virtually seamless trade with the South.
But the unionists also do not want to see UK customs, immigration or border controls set up outside the North, for instance in the ports and airports in Britain.
Elsewhere, Mr Bruton warned about experienced civil servants being diverted from their work to focus on Brexit negotiations.
"It will, I regret to say, involve the diversion of top level official talent, in 28 capital cities, away from anticipating the challenges of the future, and instead towards reopening agreements made over the past 44 years," he said.
"Our most talented civil servants will be taken up with digging up the past, rather building the future. It is a tragedy.
"The Brexit process will not be like a member leaving a club of which he or she no longer wishes to be a member, which is an easy enough process, once the bar bill has been settled.
"It will be much more like a divorce between a couple, who have lived together for years, have several small dependent children, a mortgage, and a small business they had been running together.
"Not only have past bills to be settled, but future liabilities have to be anticipated, decisions made about the running of the business, and rights and responsibilities in respect of the children agreed."
Mr Bruton also highlighted the issue of European powers sharing information on terrorism in a post-Brexit world.
He said access to intelligence currently passed between EU nations may not be automatic, particularly if the UK rejects the jurisdiction of the European Court of Justice on disputes about what is shared.
Mr Bruton also said the DUP and Sinn Féin have revived the issue of territorial sovereignty.
And with weeks of power sharing talks looming at Stormont, he added: "In the past, Prime Ministers and retired statespersons could fly into Belfast, to provide cover for a new compromise between the parties that allowed them to get back to work.
"As Brexit will absorb so much of everyone's time in coming years, the scope for this sort of high-profile counselling will be less. Reality therapy may be needed."