Chief Medical Officer Dr Tony Holohan has said the new variant of Covid-19 in the UK has been detected in Ireland.
Dr Holohan said that further testing in the coming days and weeks would establish the extent to which it is present here.
He urged people to avoid social contact and all forms of non-essential travel.
The Department of Health was notified of 1,025 new cases of the coronavirus today and two further deaths linked to the disease.
There has now been a total of 2,194 Covid-19 related deaths in Ireland and 84,098 confirmed cases.
There are 25 people in intensive care, which is up three from yesterday; two were admitted in the last 24 hours.
Dr Holohan said the new UK variant of SARS-CoV-2 had been detected by whole genome sequencing at the National Virus Reference Laboratory in UCD.
"Further testing in the coming days and weeks will establish the extent to which it is present here. In the meantime, it is vitally important that we each stay at home, avoid social contact and avoid all forms of non-essential travel," he added.
"It is particularly important that those who have arrived from the UK strictly self-isolate for a full 14 days following arrival. They should not interact with others, visit others, socialise or go shopping.
Dr Holohan said the HSE is making arrangements to test recent arrivals from the UK.
Earlier, Dr Holohan renewed his appeal for people to limit their contacts as much as possible over Christmas.
He said none of the indicators of the virus are showing encouraging signs and Ireland now has the fastest growing incidence rate in the EU.
The best present people can give to their loved ones this Christmas is to strictly adhere to the public health advice, he added.
Yesterday, the first of the new Level 5 restrictions came into effect with restaurants, gastropubs and cafes closing except for takeaway and delivery.
Hairdressers, beauticians and barbers were among the other businesses to close until at least 12 January.
As part of the measures religious services will move online after today but places of worship can remain open for private prayer.
Meanwhile, Minister for Health Stephen Donnelly last night confirmed that he has signed the regulation authorising the use of the Pfizer BioNTech vaccine in Ireland.
The first delivery of almost 10,000 doses is due tomorrow - with the first vaccinations on 30 December.
You can make your own Christmas safe, meaningful and memorable for the right reasons. Stay home, stay safe and stay well. Limit contact as much as you can. Mind yourself. Watch out for others. Follow our public health advice. Have a very happy Christmas. @roinnslainte @HSELive— Dr Tony Holohan (@CMOIreland) December 24, 2020