The chair of National Public Health Emergency Team (NPHET) Advisory Group says we would have faced 'hundreds' of coronavirus cases in a few weeks' time, if nothing had changed.
It comes after updated public health advice from the Government on the back of NPHET - which includes those over 70 being encouraged to limit their social interactions to a small group of people.
While visits to homes should be limited to six people from no more than three households.
Professor Philip Nolan told Newstalk Breakfast: "What we know is that the virus is spreading, that it's spreading between work places and households and increasingly from households into the community.
"And we know that the reproduction number is above one: in the region of 1.4 to 1.8 - and that means that over the coming weeks, if noting changes, we will see a very significant increase in the number of cases over the next two to three weeks - over and above the increase that we've already seen.
"But the good news is that we know that we can suppress this virus, we've done it before, and we know that the measures around physical distancing, hand hygiene and in particular the thrust of the new measures to reduce the amount of contact between households.
"It's an unfortunate place to be, that again we have to reduce our social contacts which means a lot in our lives.
"But we do know that if we reduce the level of contact between households in these kind of gatherings and congregated settings that the new measures are targeted at that we can suppress the virus.
"So on the one hand our models show us that if we do nothing or carry on as we are, we would be in a very serious situation in two to three weeks' time.
"But equally, and perhaps more importantly, experience shows us that if we take on the spirit and the letter of these measures that we can bring the growth of the disease down and that we can suppress the disease".
"We're certainly looking at hundreds of cases in a few weeks' time if we hadn't taken the action that the Government took last night and that NPHET recommended yesterday".
"There's an enormous amount of work being done to control the outbreaks in those congregated settings - which is meat factories and those kinds of production settings.
"There was a very significant amount of work done to set up the testing and tracing system that we have.
"It has shown some strain over the last week, and a huge amount of work is now being done to restore the turnaround time back to under 72 hours."
On the measures announced on Tuesday, Prof Nolan denied they were targeting people who may be doing the right thing.
"I don't think it's any intention to punish - and it's been said before: the virus has no value system to value judgements.
"All it wants is an opportunity to infect people, and that means the only way we can prevent its transmission is for all of us - and we're not singling out any group here, and have no intention to single out any group.
"In fact, quite the opposite: what we're doing here is appealing to the adults in our community to radically reduce their contacts, so that we can facilitate the children in our community getting back to education".
On training and sports, he said: "There is empirical evidence both here and internationally about the sort of congregations that we know are occurring."
He said there was "no issue here about sporting activity and training", but added: "We need to be clear that it doesn't matter what you are doing - whether it's playing music or playing sport - once the group size gets significantly above 10 to 15, you're going to run into very significant problems with household mixing.
"So there is a real consistency here: the message here is that if you're indoors, it should be no more than a group of six - if you're outdoors, it should be no more than a group of 15".
