The source of the Republic's only case of foot and mouth, which occurred at Proleek Co. Louth in early April, is being investigated by Gardaí who suspect it was carried across the borderby a truck which passed through the infected farm at Meigh in Co. Armagh.
The suspected truck is thought to have been used for the illegal transportation of animals and results of tests for the virus on the truck are expected shortly.
Meanwhile, the return of the virus to the island of Ireland yesterday prompted the Minister for Agriculture, MrJoe Walsh, to order stricter border controls in a bid to prevent the disease spreading.
He said the case in Co Antrim amounted to a "extremely worrying" development.
The Minister appealed to all farmers to contain their premises and ensure foot-and-mouth disinfecting procedures were rigidly observed.
"This fresh outbreak means that there is a geographic spread of the disease, and not alone is it active on the island of Ireland, but it is actually rampant," he said.
"So we have to literally redouble our efforts. We have to remain vigilant, to have each farmer to have their farm as a fortress, as a self-contained unit - to make absolutely certain that the disinfection and hygiene facilities are in place, and that along the border with Northern Ireland there is no movement of livestock or animal products in any way.
"This is a salutary reminder that we have a long way to go to win this battle."
In another development, Sinn Féin revised arrangements for events at the border marking the anniversary of the 1916 Easter Rising, in response to the foot-and-mouth crisis.
Some speakers and bands due to attend a commemoration in Monaghan were requested not to travel after Sinn Féin consulted veterinary officials.
Caoimhghín Ó Caoláin, TD, said: "Increased vigilance on the part of all citizens on both sides of the border is now required."