There are up to around 4,000 positive coronavirus tests in the current system which have yet to be reported in official figures, NPHET has said.
In a press briefing on Thursday evening Prof Philip Nolan Chair of the NPHET Irish Epidemiological Modelling Advisory Group said that in recent days very few people were presenting for tests over the Christmas period.
And Prof Nolan revealed some 4,000 tests are in the system that are yet to be confirmed as positive cases.
Speaking on Thursday he said they estimate there is "somewhere in the region of 4,000 positive tests" reported in the past 2-3 days which have yet to be formally confirmed as cases.
"Simply put, this system was not designed to report 2/3/4,000 tests per day, and it is going to take several days to confirm all of those cases,"
Prof Nolan also warned that NPHET was expecting a "significant further increase in cases and hospitalisations in the coming days before public health measures take effect and are likely to see over 700-1000 people in hospital with COVID-19 early in the New Year."
He said: "The epidemiological situation is very serious and the virus is spreading rapidly amongst all age groups, increasing the risk to those most vulnerable to severe infection, such as the medically vulnerable and people over 60 years of age.
"The reproduction number is at least 1.6-1.8. The daily growth rate is estimated now at 7%-10% giving a doubling time of 7-10 days or less."
It comes as a further 1,620 new cases were confirmed while 12 deaths were announced.
Speaking on the latest figures Chief Medical Officer Dr Tony Holohan said that NPHET was no longer advising close contacts of confirmed cases to get tested.
Instead they will be told to isolate for 14 days.
