Covid levels among 18 to 34 year olds have increased by up to 70% over the last few days.
The Health Minister has also expressed concern that people with symptoms are not self-isolating.
Over a quarter of 19 to 24 year olds going for a PCR test are now getting a positive result.
NPHET's head of modelling, Professor Philip Nolan, says it's the first sign of the Omicron variant spreading rapidly.
Health Minister Stephen Donnelly says case numbers will start rising very soon, while infectious diseases expert Sam McConkey says younger people are yet to be offered a booster.
The Chief Medical Officer has raised concerns that only half of people with symptoms are isolating at home. Head of the ESRI's behavioural research unit, Pete Lunn, says antigen tests may be playing a role in that.
Health officials are urging everyone to cut down on socialising - and for people with symptoms to self-isolate and get a test.
