The Department of Health has been notified of a record 20,554 new cases of Covid-19.
Chief Medical Officer, Dr Tony Holohan, said that the Omicron variant now accounts for over 90% of PCR confirmed cases in Ireland.
According to today's figures, there are 619 people in hospital with the virus, a rise of 51 since yesterday. Of these, 88 are in ICU, which is down five.
The 14-day incidence of the virus is now estimated at over 2,300 per 100,000 population.
"In the last 24 hours, 148 people with Covid-19 were either admitted to hospital or received a 'detected' test result while in hospital," Dr Holohan said.
He said "hospitalisations at this level are not sustainable and are having a significant impact on our health service".
He urged people not to hold household gatherings on New Year's Eve and called for people to limit their social contacts.
Meanwhile, Deputy CMO Dr Ronan Glynn said that the incidence of the disease across the country is "now much higher than at any point in the pandemic.
"Because of this, many people, despite being boosted and having taken other measures to protect themselves will be infected with SARS-CoV-2. However, their booster will protect the vast majority from serious illness or hospitalisation, and it will help to protect our health service and critical care capacity."
The latest figures comes as the Minister for Health gave updated advice in relation to community testing and the period of isolation for those diagnosed with the virus.
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Stephen Donnelly said that symptomatic people aged 4-39 years should self-isolate immediately and undertake regular antigen tests rather than booking a PCR test. He advised that if Covid-19 is detected in the antigen test, then a confirmatory PCR test should then be sought.
Anyone who received a booster vaccine and gets Covid will have to isolate for seven days, rather than the current ten days.
In Northern Ireland, the Department of Health reported 4,701 positive Covid cases and three deaths in the last 24 hours.
It comes as the Stormont Executive agreed to reduce the isolation period for positive cases from 10 days to 7, subject to negative lateral flow tests on days six and seven.
The Omicron variant is now responsible for more than 90% of Covid cases in Northern Ireland.
Testing system is 'maxed out'
The true number of Covid-19 cases is much higher than daily reported infections, according to an associate professor in the Department of Clinical Medicine at Trinity College.
Dr Clíona Ní Cheallaigh, who is also a Consultant Physician at St James's Hospital in Dublin, pointed out that the testing system is maxed out.
Speaking on RTÉ's Morning Ireland, she said it is sensible to assume that if a person has symptoms that seem like Covid or have a positive antigen test, then they do have Covid and should follow public health advice accordingly.
However, she added that there are a number of reasons why people want to know if they have a positive PCR test, including for work purposes and this was "challenging".
No PCR test slots available
There were no PCR Covid-19 test slots available anywhere in the country at lunchtime today.
David Cullinane, Sinn Féin's health spokesperson, said a "staggering 6% of the population is being tested a week" with the Health Service Executive approaching full capacity.
"This is causing huge difficulties for people in getting access to a PCR test. Clearly Omicron is rampant in the community."
Mr Cullinane called for antigen tests to made freely available. He also urged the Government "to set out a strategy on how we manage and respond to [high cases] ... specifically in relation to testing and public health messaging and hospital capacity."
RTÉ News understands that the Cabinet will meet next week, possibly on Wednesday. The Government's position on schools is believed to remain unchanged and they are set to reopen after the Christmas holidays.
Call for activity in hospital system to be curtailed
The Irish Nurses and Midwives Organisation has called for all activity except urgent emergency care to be curtailed in Ireland's public hospital system.
The INMO's General Secretary Phil Ní Sheaghdha warned: "We are yet again seeing high numbers presenting to Emergency Departments across the country with 287 patients on trolleys today.
"From experience we know that if there are 287 patients for whom there is no bed in a hospital on 30 December, we know that figure will be tripled in early January.
"It is time now to scale back all activity within our public hospital system to emergency activity only."
Also today, the Department of Public Health Mid-West is advising people in Limerick, Clare, and North Tipperary to limit social contacts during New Year's Eve celebrations.
It warned: "Indications are that the number of daily Covid-19 cases in the Mid-West are more than double what they were before Christmas."