The strain meant UK Prime Minister Boris Johnson had to cancel Christmas plans for millions of people as it took hold in England.
The faster-spreading COVID-19 variant believed to behind a surge in cases in England has been detected in Ireland and Northern Ireland, officials say.
This relates to the coronavirus strain first revealed by Downing Street last week, named VUI-202012/01 – not the second variant that has since been linked to contacts of travellers from South Africa.
VUI-202012/01 has been in the Republic since at least the second week in December, according to data from samples taken last weekend, the National Public Health Emergency Team (NPHET) said in a statement.
Officials in Northern Ireland say only one case has been found so far, based on a handful of suspected samples.
On Saturday, concerning data regarding the new variant saw Prime Minister Boris Johnson cancel Christmas for millions of people in England after he was advised that it was up to 70% more transmissible.
It has been especially present in London and the southeast of England, but is believed to have spread further and has been found in Scotland and Wales, as well as some mainland European countries.
Northern Ireland is the last UK nation to record a case.
The country’s health minister, Robin Swann, urged people to “avoid panic and complacency” ahead of a six-week lockdown starting from Boxing Day.
“This lockdown will only work if we all fully play our part – strictly follow the COVID regulations and public health advice and be ultra-careful in everything we do,” he said.
“I would urge everyone to review their plans for Christmas and to err on the side of caution. Just because you can do something, it doesn’t mean you have to.”
The Republic of Ireland banned arrivals from the UK in response to Mr Johnson’s announcement, with all passenger ferries and flights barred until at least 31 December – although “mercy flights” did get some Irish nationals home.
Around 30,000 people have travelled to the Republic from the UK in the last fortnight, largely to see relatives over the Christmas break.
Its health body added that the emergence of the new strain is not solely responsible for a rise in infections in Ireland, which is also preparing for another lockdown.
NPHET said Ireland’s R number, the rate at which the virus grows, is between 1.5 and 1.8 – the highest since March.
“Every indicator of the disease is rising and rising rapidly. Our level of concern continues to escalate,” Ireland’s chief medical officer Tony Holohan said in a statement.
From Thursday, pubs, restaurants and some non-essential retail in Ireland will close in another lockdown, which could last several months – however scientists worry these restrictions may not be enough.
Philip Nolan, the head of Ireland’s COVID-19 modelling group, has warned that daily cases will average around 2,000 unless more stringent measures are taken.
“Given the nature of the measures and our experience to date, it’s hard to see how that array of measures would bring reproduction back down below 1,” he said.
It comes as the country reported 938 new cases of coronavirus on Wednesday, and a further 13 related deaths.
Ireland has reported a total of 82,155 cases and 2,184 deaths since the pandemic began, according to figures being tracked by Johns Hopkins University.