Two members of staff at the Allianz insurance company headquarters near Elm Park in Dublin have contracted Legionnaires' disease.
Following tests, the company says a cooling tower used as part of the air conditioning system was found to have high levels of the Legionella bacterium and that the tower may have been the source of the outbreak.
The cooling towers were shut down immediately, and the Health Service Executive and Health & Safety Authority were also notified.
One of the staff members is in his late 40s, the second is in his mid 50s.
The first case was diagnosed on 27 June and the second yesterday evening.
Assistant Director of Risk Management at Allianz James Gleeson said that both men are being treated at the Mater Hospital.
A further five employees have been tested in the last 48 hours as a precautionary measure.
The five people were examined by hospital doctors before being released.
Final test results will be known tomorrow, but it is not thought that any of the five have contracted the disease.
Legionnaires' disease is a type of pneumonia and is quite rare with less than ten cases reported in Ireland each year.
Allianz says it has taken all steps necessary to guarantee that any further risk of infection has been neutralised.
The company says that all employees have been informed about the outbreak.
All managers have been asked to contact staff who are out of the office, to make them aware of the next steps to take if they have any concerns or queries.
The disease does not spread from person to person but is spread through the air from a water source.