The new Covid strain could be the invisible enemy among us fuelling the rocketing spread of the disease.
Preliminary data has found it in the east of the country.
The new strain, which is believed to be much more infectious, may not only be driving the virus among some adults; it has been suggested it could also be easier for children to catch.
The possibility was raised by WHO envoy David Nabarro and some UK scientists yesterday.
However, like much about this strain, so full of un- knowns, more investigation is needed.
If it is more transmiss- ible among children it could have implications for schools re-opening.
Hot on its heels yesterday came news of another mutant strain from South Africa, which is even more infectious.
It is thanks to the expertise of scientists at COG-UK (the Covid-19 Genomics UK Consortium) that we have been alerted to the new strain.
They are leading the way in trying to analyse its impact.
Prof Neil Ferguson, an epidemiologist in Imperial College London, suggests the new variant may make children more like adults in the infection and spread of the virus.
The re-opening of schools here since September has been a success, due partly to the low transmission rates among children.
COG-UK says it is not familiar with any data now to support that suggestion.
If it is found to spread more among children, it will lead to questions about re-opening schools after Christmas.
Chief Medical Officer Dr Tony Holohan remained strongly of the view yesterday that schools will re-open, saying it is a core priority.
The biggest threat would be high levels of community transmission.
Recent weeks have seen a number of school outbreaks, although they remain low.
Schools have continued to operate without interruption since September, even during the last escalation in spread in October and the six-week lockdown.
Around 30,000 people have arrived from Britain in the past two weeks, and the health authorities here are putting them in the spotlight.
GPs have been alerted, and anyone who arrived from Britain in the past 14 days is being advised to self-isolate - to not mix with any other household members, leave the house or see visitors for a full 14 days after arrival.
This is because they could be incubating the new strain of the virus.
All will be contacted by the HSE via text message from the contact details they put on their passenger locator forms.
They will also be advised to contact a GP or out-of-hours service to organise a free Covid test, whether or not they have symptoms on day five after arrival, or as soon as possible after that.
Even if their test is negative or not detected, they must self-isolate for the full 14 days.
In particular, the HSE is concerned about young and old people mixing during the holidays.
Many will already have had a private test, and irrespective of whether this is negative or non-detected, they must go into self-isolation.
GPs were told to be on high alert for people with symptoms who have returned from Britain and to be vigilant about infection prevention and control measures when assessing or providing care to them.