Thirty-nine farms have been sealed off in three western counties and 300 sheep will be slaughtered in Mayo today as part of the campaign against foot and mouth disease.
The animals are being destroyed after it emerged that they had been transported from England two weeks ago.
They are believed to have been brought into the North at the same time as the 248 lambs which ended up at the Kepak plant in Athleague, Co Roscommon.
It is understood ear tags, which are necessary for sheep in the North and Britain, had been removed.
The Department of Agriculture insists the measures were purely precautionary.
The slaughter is the fourth such operation in the past week, with similar culls in counties Louth, Wexford and Carlow.