By Michael McHugh and Aine McMahon, PA.
Eighty five new cases of Covid-19 were diagnosed on Thursday in Ireland in a worrying escalation, health chiefs said.
A factory in Kildare and at least two direct provision centres in the Midlands where some of the workers lived have created a cluster of infection.
Construction sites were also connected to the sudden increase, health officials said.
Acting chief medical officer Dr Ronan Glynn said: "We are very concerned."
He said 68% of those diagnosed were aged under 45.
"I hope this is a blip, I hope we see a much smaller number of cases tomorrow," he said.
He said the factory in Co Kildare had seen 30 to 40 positive cases.
Not all tests have been returned yet.
A total of 18 of Thursday's tally were associated with the factory.
"We need to watch this extremely closely, we need to see in particular what happens tomorrow and the next day.
"Now is not the time for a knee-jerk reaction.
"We do understand where the vast majority of these cases have come from.
"The question now is what happens next."
One more death was reported on Thursday by the National Public Health Emergency Team.
Infection control experts are working with the direct provision centres.
The Kildare factory has been closed.
A smaller number of cases were associated with the construction industry and others with households and extended families.
Over the last 14 days 331 new cases have been notified.
Dr Glynn said: "Today's figures demonstrate how quickly Covid-19 can remerge in our country.
"We are now at a crucial point in our response to Covid-19."
Ireland has announced a five-stage plan for relaxing restrictions designed to limit spread of the disease, but dates for some reopenings have been pushed back as the pandemic worldwide shows now sign of abating.
Professor Philip Nolan, chair of the Irish Epidemiological Modelling Advisory Group, said: "Over a two-day period Ireland moved from a relatively stable epidemiology to a significant pattern connected to outbreaks.
"We now need to be really careful and adhere to public health advice so we do not further spread the virus."
Tanaiste Leo Varadkar has said he hopes pubs that do not serve food can reopen for business on August 10, but there are no guarantees.
Pub lobby groups have called on the Government to release guidelines that will allow them to reopen safely, after their previous reopening date was delayed due to an increase in coronavirus cases earlier this month.
Dr Glynn said it was too early to say when a decision would be taken.
Hospitality group Failte Ireland is expected to publish reopening guidelines for pubs in the coming days.