Tom Parlon, president of the Irish Farmers' Association, said the foot-and-mouth outbreak was "the worst possible news - a major body blow for the whole country".
Until suspect sheep were located in Co Louth, there had been a feeling that Ireland was escaping the virus, he went on.
"The link would still appear to be very much with imported sheep, the original infection that was hibernating and growing beneath the surface.
"I think there will be an immediate slaughter of all the sheep and stock in the area. That is the only way we can deal with it, as an absolute emergency," he said.
"Farmers will support all measures that are necessary. Confining it to an exclusion zone is now the priority."