Two members of Dublin Airport's fire service were admitted to ICU after up to 18 staff tested positive for Covid-19.
It is understood that a man in his 50s, from the east of the country, is still in intensive care and the other has been discharged since a cluster of cases came to light.
Members of the airport authority's fire crew in Cork were brought in to provide additional cover and maintain minimum levels of staffing. They worked from a separate base at the airport.
"We are aware there's been a number of confirmed cases in the fire station and obviously it is very concerning," said Siptu aviation sector organiser Neil McGowan.
"We are engaging with the employer to ensure all social distancing measures have been put in place.
"People have done a fantastic job in very difficult circumstances to keep the airport open and keep essential medical supplies coming into the country.
"That includes firefighters coming from Cork to supplement the crews to ensure they have the numbers."
He added a minimum number of staff had to be available for aircraft to land, regardless of the number of flights. As well as providing a fire service, the crew is on standby to deal with any incidents.
A Dublin Airport Authority spokesperson confirmed that a number of Dublin Airport fire service personnel had contracted Covid-19 in recent weeks.
"Thankfully, the symptoms were mild in almost all cases and almost all of our fire service colleagues have either recovered or are recovering," he said.
He added some had returned to work.
"It would not be appropriate to comment on any individual cases relating to the health of our employees," he said.
"The health and safety of our employees, our passengers and other airport users is always Dublin Airport's main priority. Throughout this public health crisis, Dublin Airport has adhered to all of the Government's public health guidelines."
The HSE's Health Protection Surveillance Centre was notified of 15 workplace clusters or outbreaks of the virus by April 17. The highest number were in the south, but overall workplace clusters make up just 3pc of all outbreaks.