The chief medical officer has blamed a "substantial amount" of socialising in December for the sharp rise in Covid-19 cases over Christmas.
Dr Tony Holohan said the country was now at a point of "unsustainable growth" in the level of infections.
His comments came as Ireland re-entered Level 5 restrictions due to a sharp increase in the spread of the virus and a significant increase in hospitalisations in the past week.
A record daily figure of 1,718 new cases of Covid-19 were reported by the Department of Health on Wednesday.
There were 13 additional deaths linked to the virus.
Dr Holohan told RTÉ's Morning Ireland programme: "We know a substantial amount of activity took place in the days and weeks following that easing of restrictions.
"We know that the average number of contacts - we track this kind of information - that each case is identifying has increased sharply to levels that we haven't seen since the beginning of this pandemic.
Dr Holohan said the country was now at a point of "unsustainable growth in the level of infections, albeit still lower levels than the rest of Europe is experiencing, but one that really creates a risk".
He appealed for the public to take action in a bid to change the patterns of transmission.
"Everybody listening this morning has an opportunity now to look at their behaviour, to look at their plans over the course of the next number of weeks, as we head in through the new year tonight, and then into the early days of the new year with the resumption of schools on January 11, we have an opportunity to take actions that can turn around these patterns of transmission," he said.
He added that if everyone adhered to the latest public health measures and stayed at home except for exercising within a 5km radius of their home and essential reasons that the cycle of infection could be broken.
Taoiseach Micheál Martin described the state of the pandemic as "extremely serious" as he announced the introduction of stricter measures for at least one month in a televised address to the nation on Wednesday evening.
The new measures include a ban on visitors in private homes or gardens unless they are providing care to children or the elderly or vulnerable, or part of a support bubble.
No social or family gatherings in any setting will be permitted, with an exemption for weddings with up to six guests, and funerals with up to 10 mourners.
The public is being asked to remain at home, apart from travel for work, education or other essential purposes.
Exercise is permitted within 5km of home, but all non-essential retail and gyms will shut their doors at close of business on Thursday.
Schools will reopen on January 11, extending the Christmas break by three days.