Anti-Brexit protesters brought traffic to a crawl on the main road between the Republic of Ireland and Northern Ireland on Saturday.
A convoy of demonstrators staged a noisy motorway go-slow near Dundalk in Co Louth using lorries and tractors to highlight the impact of predicted customs checks on the local economy.
Contingency work has begun to identify places where checkpoints could be set up once again, in preparation for the UK leaving the EU, the Government said.
Since the end of the conflict traffic has passed largely unimpeded between the neighbours on what would be the UK's only land border with Europe.
A trailer-load of sheep going to market and passport-toting residents took part in a theatrical “check point” staged to highlight the detrimental impact of a Border with the North.
A rusty second World War-era bicycle placed alongside a mock customs hut reinforced the message that a hard Border was a return to the past which British prime minister Theresa May has been adamant she wants to avoid.
Residents said they were caught in the eye of a storm which could have a devastating impact on relations between the North and the Republic.
Demonstration organiser for Border Communities Against Brexit, Declan Fearon, said: "We are really in the eye of the storm of Brexit and we intend to make sure that this does not happen.
“We never want to see this community going back to what it was before.”
Mock border officers from the UK and Ireland wearing traditional greatcoats waved down traffic in front of a stage customs hut.
Horns blared, truck drivers in the queue pretended to pull their hair out and waved pieces of paper supposed to be travel documents. Lines of people with placards had gathered. Vintage signs read: “Stop: Customs”.
Mr Fearon said: “The people here do not want to contemplate the reinstatement of spikes and roads being closed and of customs check points and it looks like that is where we are going.
“It seems as if Theresa May and the British government have no intention of listening to us.”
He said: “They would not contemplate any assurances or guarantees that the Good Friday Agreement would not be injured or changed in any way as a result of Brexit.
“They don’t seem to have any of our interests here along the Border at heart and we want to make sure our voices are heard as far and wide as possible.”