Brexit will more radically change Ireland's relationship with the UK than the 1916 Easter Rising and partition, an MEP said.
Mairead McGuinness, vice president of the European Parliament, warned of the threat of deep divisions across the Irish Sea as she launched a broadside over Prime Minister Theresa May's proposals for the split from Europe.
The MEP with Ireland's ruling Fine Gael party said ideas on customs, trade and the Irish border "are more than the UK wanting to have its cake and eat it, it's an attempt to have its cake and eat ours".
Ms McGuinness made her remarks at the annual Beal na mBlath commemoration in west Cork on the 95th anniversary of the killing of Irish rebel leader Michael Collins.
And she hit out at calls for the UK's split with Europe to be used as a means for reuniting Ireland.
"There are those who would use Brexit as a weapon to reunite our country. That is misguided. The path to reunification is already set out in the Good Friday Agreement," she said.
Ms McGuinness said Brexit has the potential to impact more on Anglo-Irish relations than Ireland's War of Independence or the Declaration of the Republic in 1949.
"If the shape of Brexit is a hard one then the separation will be more definitive and absolute than anything envisaged by those involved in the foundation of the state, including Michael Collins," Ms McGuinness said.
She warned about the impact of calls for the UK to leave the European customs union adding that each time "another brick gets placed back in the border wall".
Ms McGuinness also called for a national debate to take place in Ireland on the future of Europe.
"Brexit, as profound as it is, must not be allowed sap all energies and efforts," she said.
"We need to start looking beyond Brexit to what type of Europe we want to see in the future.
"Scepticism and indeed cynicism about the EU was certainly fuelled by the economic crisis.
"And mistakes were made giving rise to justifiable concerns among citizens which must be addressed in any discussion about the future of the EU."
Ms McGuinness urged all elements of society to get involved in a debate on the shape of Europe in the future.
And she raised concerns about the breakdown in powersharing in Northern Ireland.
"Brexit has reopened hardly-healed wounds of division and deepened the polarisation of politics in Northern Ireland.
"The decision of the British Government to accept the support of the DUP to remain in power has added sharply to that polarisation," she said.
Ms McGuinness also called for Europe to start building new relationships with the Islamic world and with Africa in a bid to tackle the refugee crisis.