A variant strain of the coronavirus, first detected in the UK, is present in Ireland.
The director of the National Virus Reference Laboratory, Dr Cillian De Gascun, says preliminary data suggests this.
He says this is based on a selection of samples analysed from the weekend.
"Preliminary data would suggest, based on a selection of samples analysed from the weekend, that the novel variant from the UK is present in Ireland. "
But he says that this is not the only reason for higher cases.
"Given the timeline of the samples analysed, it would seem that the novel variant is not solely responsible for the recent increase in case numbers seen in Ireland."
While Northern Ireland's Department of Health has confirmed the presence of the new strain in the region.
It says the new variant was found in a positive test on Wednesday.
Genome analysis had been carried out on a small number of suspected cases there, producing one positive result.
The department says the variant is likely to have been present in the North "for a period of time."
It comes as there have been 13 additional deaths related to COVID-19 and 938 further confirmed cases of the virus in Ireland.
Some 300 cases are in Dublin, 110 in Cork, 72 in Limerick, 68 in Donegal and 41 in Kildare.
The remaining 347 cases are spread across 21 other counties.
Chief Medical Officer Dr Tony Holohan has said people should revise their Christmas plans to ensure social contacts are limited.
While Deputy Chief Medical Officer, Dr Ronan Glynn, said: "The epidemiological situation reviewed today is the most serious it has been since last March.
"People should act at all times as if they or those they come into contact with are infectious.
"The disease has spread across all parts of the country and all age groups, we must act now to protect each other."
"It is inevitable that people will get sick and die as a result of this escalation, but it is not too late for all of us to do all we can to minimise that impact and to protect as many people as possible."
'Serious and dangerous position'
Earlier, HSE CEO Paul Reid said the health body was working on the 'absolute premise' that the new strain was already here.
Concerns over the new strain have prompted the Government to ban travel from Britain until the end of the year.
Mr Reid told Newstalk Breakfast: "There is no doubt about it - we're in quite a serious and dangerous position in terms of the transmission of the virus right now.
"Certainly what we have seen is a higher number of contacts... we can see people are meeting each other more, out more, shopping and visiting because the opportunity is there. That certainly is a factor."
He said checking for the new strain is now part of the testing process.
"We haven't identified it to date, but that doesn't indicate it isn't here. We're working on the absolute premise it is here."