Professor Philip Nolan of the National Public Health Emergency Team (NPHET) has said it only takes a marginal change in people's behaviour to bring Covid-19 "back under control".
The Chair of NPHET's Epidemiological Modelling Advisory Group said their projections in an "optimistic scenario" of a possible 200,000 cases in December is not inevitable.
Speaking on RTÉ's Morning Ireland, he said those models are based on what might happen if nothing changes over the coming weeks in terms of people's level of social contact and the care they take to prevent transmission during that social contact.
Over the last two to three weeks, he said, daily cases have gone from 2,000 to 4,300 and he warned that if nothing else changes Ireland is on a "trajectory towards very large case numbers and very large numbers of people in hospital coinciding with Christmas".
The risk that 450 people may need intensive care by Christmas is "possible" but he said they "are trying to do everything we can do avert that scenario".
He said those models have only just been run and they will rerun them again next week.
There was a huge growth in cases in the immediate aftermath of 22 October and through the midterm break, which coincided with "very intense socialisation" across all age groups, he added.
There was a huge growth in cases over the last two weeks, he said.
"It seems slightly less this week. Right now, we will be tracking the more optimistic end of those models."
He said it reinforces the message that marginal changes in behaviour in a positive direction can bring the virus under control.
"The fundamental message here is we need to act … we need to self isolate if we have symptoms … we need to reduce our contacts, perhaps only by a third or improve our risk mitigation during those contacts."
Prof Nolan said they are not surprised by this wave of the virus as it "was one of the possibilities that was envisaged beyond October 22".
"It puts us in a position where right now we have to send the signal that unless we can pull back in our level of contacts, work from home and be very careful about our priority contacts, we are heading for a very challenging December."
He said it is entirely possible to bring the virus back under control if public health advice is followed and allow the booster programme "to do its job through Christmas and into spring".
A reproduction number of 1.2 or 1.3 is not that far above 1, he said, but is "enough to sustain a rapidly escalating epidemic".
Mr Nolan said it is impossible to predict or prejudge the "very small changes in our collective behaviours", which he described as the difference between the virus being suppressed or marginal changes in the other direction.
ESRI looks at changing behaviour during pandemic
People have doubled the number of people they are meeting outside their household since the start of the year, the Economic and Social Research (ESRI) has said.
Data from the ESRI's Behavioural Research Unit shows that people have moved from meeting around two others outside their household in January compared to four others now, Dr Deirdre Robertson said.
Speaking on RTÉ's Morning Ireland, Dr Robertson said one "of the biggest predictors" of social activity has been the level of worry over the virus.
"As cases have gone up, worry has gone up and that has changed behaviour."
While there was still "significant worry" among the public, it is not increasing at the same level as it did last year when cases were rising, she said.
"Probably since the vaccination programme kicked in, that accounts for some of the drop."
People are worried about the healthcare system, protecting the economy and ensuring their loved ones are safe from Covid-19, Dr Robertson said.
ESRI data at the end of October showed "for the first time" an increase in the number people saying they thought the response to Covid-19 was "insufficient", she said.
The Behavioural Research Unit has "noted" in the past few months, "along with an increase in social activity there has been a reduction in mitigation behaviours such as mask wearing and social distancing," Dr Robertson said.
"The perception of the wearing of masks and ventilation was not as strong as it should have been."
She suggested that "talking about what can be done in safe ways is as important as talking about what can't be done in order to reduce the spread of cases".