11 others are self-isolating while awaiting results.
Construction is continuing at the new Glanbia factory in Portlaoise after a worker tested positive for COVID on Tuesday.
SISK says the employee of a sub-contractor had eleven close contacts, who are now self-isolating, awaiting test results.
The site remains operational, with safety protocols in place, including daily temperature checks.
Glanbia Cheese is investing 130m euro in the plant and confirms none of its staff are effected.