The newly-discovered Brazilian mutation of coronavirus in Ireland may be more transmissible, a health chief said.
Three cases have been detected in the Republic for the first time.
All are directly associated with recent travel from the South American country, the authorities said.
Dr Colm Henry, HSE chief clinical officer, told RTÉ Radio 1: "It may well be more transmissible.
"As to whether or not it is more resistant to the neutralising antibodies that is created by vaccines, we don't know yet.
The cases are being followed up by public health teams and enhanced measures have been put in place.
The Brazil variant could be much more contagious or easy to catch than the original version of coronavirus.
It has undergone changes to its spike protein - the part of the virus which attaches to human cells.
It first emerged in July.
Around 90% of Covid-19 cases in Ireland are associated with the UK variant.
Numbers of new infections and pressure on hospitals have eased and the death toll has begun to abate following weeks of tough curbs.
The emergence of new mutations introduces significant uncertainty in predicting when society can return to normal, public health experts have said.
The Republic recently passed the grim milestone of 4,000 dead from the disease during the latest wave of mortality.
Taoiseach Micheál Martin has warned tough restrictions to slow the spread of disease could remain in place into April.
Schools and childcare will reopen on a phased basis next month after health experts gave the green light to Government to begin easing weeks of strict lockdown.
Next week ministers are expected to update their Living with Covid plan.
Ireland has also ramped up its vaccination programme.