NPHET's warning there could be up to 15,000 cases of Covid-19 a day in early January.
That's if Omicron becomes dominant and dangerous, and social contact increases to last December's levels.
Ireland's one of 38 countries that has detected at least one case of the variant.
Dr Martin Daly, a former president of the Irish Medical Organisation, says Omicron is extremely concerning:
"We don't know yet whether Omicron is going to replace the Delta variant. If that happens, then we face a situation where we would have one wave finishing and a new wave beginning. People genuinely hope that that won't happen, but it is a possibility. And the government are right to try to plan for such an eventuality and reduce the number of cases in the community."
