The Department of Health was notified of 1,296 new cases of Covid-19 today.
There has now been 85,394 confirmed cases of Covid-19 in Ireland.
A further six deaths have occurred, bringing the total number of Covid-19 related deaths to 2,200.
Today's case number was the highest reported since the pandemic began, although more tests are being carried out than earlier in the year.
The previous high was 1,283 on 18 October.
Chief Medical Officer Dr Tony Holohan said: "For the second day in a row we are reporting more than 1,000 confirmed cases of Covid-19.
"Within a few short weeks of easing measures at the beginning of December, we have now returned to the very high levels of Covid-19 circulating in the community that we have not seen since the peak of Wave Two of this disease. This is very concerning.
"While today is a positive day with the arrival of the Pfizer/BioNTech Covid-19 vaccine, it does not mean that we can relax our adherence to public health measures.
"We must continue to stay at home, avoid social contact where possible, avoid visiting friends and family and not have visitors to our homes."
Meanwhile, in Northern Ireland, a further 20 deaths have been reported by the Department of Health. It takes the official death toll there to 1,271.
A further 998 cases of Covid-19 have also been confirmed in the past 48 hours.
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A night-time curfew is also being imposed for the first week.
The restrictions, which came into effect at a minute past midnight, will be reviewed after four weeks.
A landmark day. Our first doses of the vaccine are here: well done and fair wind to all involved. We also reported our highest daily cases: 1296. The first, a ray of hope. The second a reminder of the hard work needed to suppress the virus until the vaccine affords us protection. pic.twitter.com/PqN4bGluR3— Professor Philip Nolan (@President_MU) December 26, 2020
Latest figures from the HSE show that there were 260 people with Covid-19 in hospitals in the Republic as of 8pm last night, up four on the previous evening.
Hospitals with the highest number of patients include Beaumont with 52, Letterkenny University Hospital with 26 and Tallaght University Hospital with 22.
The figures also show that as of last night, there were 25 Covid-19 patients in intensive care, with 13 of these on ventilators.
There were 64 critical care beds available.