Genome sequencing analysis was conducted on eight samples in recent days, with one being confirmed as the Omicron strain.
Public health officials say they were fortunate to identify the case, which was in a random sample referred for lab testing.
Little is known about the Omicron variant, but it is feared it could be more transmissible than other strains.
Professor of immunology at Trinity College, Kingston Mills, says Ireland's booster campaign has now taken on new importance:
"I think what we know is that immunity drops off or wanes after even two immunisations with all of the vaccines, and that boosting is very effective for restoring that immunity. It really, you know, focuses attention on boosters right now for the whole population. And I think that the case for that is now stronger than ever."
