The Department of Health confirmed 4,799 new cases of Covid-19 as the Chief Medical Officer warned that the virus has "increased sharply" among people aged 16-34.
There are 467 patients in hospital with Covid, which is up 31 from yesterday, with ICU numbers down three to 104.
"The level of disease and positivity rate amongst young adults has increased sharply in the last week," said Dr Tony Holohan.
"One in four people in the 16-34-year-old age group that are going for a PCR test have received a 'detected' result. This is one of the highest rates since the beginning of the pandemic," he said.
"This high level of disease in young people represents a high risk to those they come into contact with who have yet to be vaccinated or receive their booster.
"Therefore, if you have not yet received your booster you are best to avoid unnecessary contact with people outside your household," said Dr Holohan.
He said this pattern of rising cases among young people is in line with what has been seen in other European countries - Norway, the Netherlands and the UK.
He added: "Our message now to these younger people is, to as much as possible, cut your social contacts.
"There's only five or six days left until Christmas - you'll be meeting up with loved ones, family, friends.
"Do as much as you can now to cut your contacts, limit your possibilities to pick up the disease before we get into the Christmas period."
"It's also a concern that our latest Amárach tracking data shows that only half of people with symptoms are isolating.
"This is the single most important piece of basic public health advice - it is vital that you isolate as soon as you experience any symptoms of Covid-19 and arrange a PCR test," said Dr Holohan.
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In Northern Ireland, 2,148 positive cases and one additional death have been reported in the past 24 hours.
Earlier today, Tánaiste Leo Varadkar said that record case numbers of the Omicron Covid-19 variant are expected in the coming days.
Speaking on RTÉ's Morning Ireland, Mr Varadkar said he is hoping and expecting that high infection numbers will not bring hospitalisations and deaths as high as earlier waves.
There is no plan to impose more severe restrictions but that it cannot be completely ruled out, he said.
Record Covid-19 case numbers are expected in the coming days, Tánaiste Leo Varadkar has said, but added that the Government are 'hoping and expecting' that it won't translate to deaths and hospitalisations to the extent of previous waves | Read more: https://t.co/RwPwO0cqQl pic.twitter.com/VGnQYZ9pw9
— RTÉ News (@rtenews) December 20, 2021
He said that the combination of the booster programme, along with mask-wearing and reducing social contacts will be enough "to weather the Omicron wave".
Ireland's "world-beating vaccine programme" is the most significant thing we can do, along with individual actions, he said.
The Tánaiste added it is expected there will be two million booster doses administered by the new year.
From 8pm today, all restaurants, bars and cafés, excluding takeaway or delivery services, will be required to shut their doors.
Indoor events scheduled earlier in the day should limit attendance to 50% of capacity or 1,000 people, whichever is lower.
Outdoor events will also be limited to half capacity, to a maximum of 5,000 people.
Wedding receptions can take place after 8pm, but with a capacity limit of 100 guests. Religious services are exempt from the restrictions.
Enhanced measures also come into play for people deemed close contacts, with those yet to receive a booster vaccine having to restrict their movements for ten days.
Expert 'not filled with optimism' that curve can be flattened
Meanwhile, the President of the Intensive Care Society of Ireland said he is not optimistic that the Covid-19 case curve can be flattened in Ireland by continuing as things stand.
Speaking on RTÉ's Today with Philip Boucher-Hayes, Dr Colman O'Loughlin said with a third of ICU capacity taken up by Covid patients, and "severe limitations" on the number of elective surgeries getting done, he was not "filled with optimism" that the curve could be flattened.
"We're squeezing the last bit out of the system to get basic healthcare … it's only squeezing in the really difficult cases. If that's where we're staying, that's still pessimistic.
"This concept of being overwhelmed and cases coming in that we can't deal with, that's what the projected figures would suggest," he said, adding that they do not have the capacity to take on the extra impact.
"We are just hopeful that all the measures will work," he said.
Dr O'Loughlin, a consultant in the Mater Hospital, said that despite a lack of significant hard data, all signs pointed to the Omicron variant being very transmissible, which was a concern.
"We are operating in a fairly evidence-free zone at the moment. We do know there is a likelihood this is a very very transmissible virus, which is borne out by how quick the variant has become dominant.
"We're still lacking hard data on how severe this virus might be in terms of causing clinical infection causing hospital infection … what we are concerned about though is that the potential is very real," he said.
Pointing to public health responses across Europe, he noted others were saying that the variant "has the potential to be very, very severe", adding that the mantra now seems to be "we should prepare for the worst and hope for the best," and that Ireland was currently preparing for the worst.
"Does that make me worried as an intensive care doctor? Potentially, yes it does," he said.
Read more: Pubs, weddings, concerts: New restrictions explained
