Anti-Brexit campaigners have brought traffic to a crawl on the main road between Northern Ireland and the Republic of Ireland.
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.
Opponents of a hard Brexit have argued it could undermine peace process measures which helped transform a militarised zone pockmarked by the Troubles and overlooked by Army watchtowers and barriers into an invisible boundary.
While minor roads were once cratered and closed to control north/south traffic - and combat a thriving black market and IRA movement - s ince the end of the conflict people and goods have 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 "checkpoint" staged to highlight the detrimental impact of any such border on Saturday.
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 Prime Minister Theresa May has been adamant she wants to avoid.
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 and v intage 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 checkpoints 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 deliberately made impassable and communities split in two.
A church near where he was standing is on one side of the border while its graveyard is on the other.
The Prime Minister has pledged no return to the heavily-secured border of the past.
Mr Fearon said: "It seems as if Theresa May and the British Government have no intention of listening to us.
"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."