The National Public Health Emergency Team estimates that 11% of Covid-19 cases in Ireland are now due to the Omicron variant.
In a statement Chief Medical Officer Dr Tony Holohan said: "That's an increase from less than 1% only one week ago."
It comes after the Department of Health announced eight more cases of the new variant have been detected in Ireland, taking the total to 18.
The CMO said they arrived at the estimation because "possible Omicron cases can be identified by the number of PCR results with 'S gene target failure' - just as we did when the Alpha variant emerged a year ago."
Dr Holohan added: "While evidence on disease severity and immune escape is still emerging, it is clear this variant is more transmissible.
"We are hoping for the best while preparing for the worst," he added.
His statement concluded: "Over the coming days and weeks global health authorities will learn more about this variant and the risks it poses. In the meantime, we continue to have confidence in the basic measures to reduce transmission."
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There have been 4,688 new cases of Covid-19 reported by the Department of Health this evening.
There are 518 people in hospital who have tested positive for Covid-19, up 14 from yesterday.
108 of these are receiving treatment in intensive care units, down one from yesterday.
Michelle O'Neill said there were 10 confirmed cases of the new strain in Northern Ireland.
Meanwhile British Health Secretary Sajid Javid has said the Omicron variant is spreading swiftly and will become the dominant variant in the British capital in the next 48 hours.
"No variant of Covid-19 has spread this fast," Mr Javid told parliament at Westminster.
"While Omicron represents over 20% of cases in England, we've already seen it rise to over 44% in London and we expect it to become the dominant Covid-19 variant in the capital in the next 48 hours."
