A sudden spike in Covid-19 infections, with a single-day rise in 85 people found to have the virus, has led to major fears about the start of a second wave of the killer disease.
It has shaken public health officials while plunging the country into a state of uncertainty about the potential grip of coronavirus.
The next few days will be crucial in determining if it is a blip and the virus is confined to clusters or if it has seeped into the wider community.
Acting chief medical officer Dr Ronan Glynn warned: "Today may be a blip associated with a number of specific clusters or it may be a sign of something more significant."
The number of new cases reported yesterday - more than double the worst-case scenario seen earlier this week - is the biggest setback this summer and comes in advance of the August bank holiday weekend and schools reopening.
Central to the upsurge is a dog food factory in Kildare which has seen a large cluster in cases and has shut down to control the spread.
Irish Dog Foods closed its plant in Naas last Friday after an outbreak was discovered.
Dr Glynn said yesterday that a number of the workers at the factory are resident in direct provision centres nearby and all have been tested for the virus.
People who have the virus have been referred to hotel accommodation to self-isolate.
The other clusters are in the construction industry, private households and extended families.
Dr Glynn said there are 26 cases in Kildare, 18 in Dublin, 11 in Clare, nine in Laois, seven in Limerick and four in Meath, with the remainder spread across seven other counties.
More than one third of new cases were close contacts of somebody who had already tested positive.
One more death from the virus was reported yesterday but the number of people being treated for it remains very low.
Asked how he would categorise his concern now, Dr Glynn said: "We are very concerned."
He added that he hoped the daily number of new cases would be back below 40 today.
Up to yesterday there were around 20 cases a day.
Dr Glynn said we need to "take a moment and see what comes through on Friday and Saturday and make recommendations if needed".
264 cases have arisen in the last fortnight, higher than the number in the previous two weeks.
"We need to understand the details between the cases. We need to understand the links between the clusters," Dr Glynn added.
He pleaded with people to adhere to the basic messages of physical distancing, handwashing and face mask wearing and said his fear is that "people think this is all over".
He said people should see everyone they come in contact with as "potentially infectious".
Asked about the reopening of over 3,000 pubs which only sell alcohol only, Dr Glynn said he acknowledged the impact of the pandemic on workers and businesses.
But he added that the reopening of schools and the risk to residential centres of a resurgence of the virus had to be part of the wider considerations.
His comments will be seen as a grim signal to the remaining publicans who are hoping to reopen on August 10.
Professor Philip Nolan, of Maynooth University who leads a team tracking the virus, 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. We must remain vigilant."
He said there are very low levels of travel-related cases and the number of deaths from the virus remains very low.
Up until Tuesday the best estimated of the R number was just below one, he added. Keeping the rate below 1 is considered key because it means the outbreak is shrinking.