A number of chicken farms in County Cavan are among eight that have tested positive for salmonella in the Republic of Ireland.
Restrictions have been placed on the affected sites.
It is understood they are in various locations around the country and culls are under way.
Officials at the Irish Department of Agriculture, Food and the Marine are investigating.
Salmonella, external can cause diarrhoea, stomach cramps, vomiting and fever. Most people recover without treatment.
The Food Safety Authority of Ireland said there were no human cases of illness linked to the investigation.
In Northern Ireland, the Ulster Farmers’ Union has urged flock keepers to be vigilant and maintain strict biosecurity standards.
A salmonella testing programme is in place in the Republic of Ireland, with flocks tested at multiple points during their lives.