Anti-Brexit campaigners have brought traffic to a crawl on the main road between Northern Ireland and the Republic.
A convoy of protesters 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 Irish 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 any such border.
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.
Local residents on the frontier between north and south said they were caught in the eye of a storm which could have a devastating impact on north/south relations.
Kitchen maker and 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 proclaimed "Stop: Customs".
Mr Fearon added: "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."
More than 20 years ago there were 270 crossings along the 300-mile long border between Northern Ireland and the Republic.
Mr Fearon said only 17 of them had clearance posts and many others were closed with roads cratered and communities split in two.
A church near where he was standing is in the north while its graveyard is in the south.
He said protesters were trying to make their voices heard in Westminster and Dublin.
The UK Prime Minister has pledged no return to the heavily-secured border of the past for Northern Ireland.
Mr Fearon said: "It seems as if Theresa May and the British Government have no intention of listening to us.
"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."