Dr Tony Holohan says there's "simply not enough data" to say for certain that the Omicron variant of COVID-19 causes less severe illness.
The Chief Medical Officer said that might be the case, but even then there would still be a risk Ireland would see "many, many more infections".
Omicron is now estimated to be the dominant strain in Ireland, with NPHET believing it's responsible for around 51% of new cases.
New restrictions - including an 8pm closing time for indoor venues - kick in today in a bid to curb the spread of the new strain.
On Newstalk Breakfast, Dr Holohan said it was necessary for NPHET to recommend new measures last week.
He said: “Restrictions are things that nobody wants to have to deal with, and we don’t want to advise. But they’re necessary because we face a major wave of infection due to the new variant.
“The scientific community has not concluded it’s less severe. It might be, but there’s simply not enough data.
“Even if it is [less severe], the risk is we will have many, many more infections… we could still see very significant impacts."
He said the good news is that booster vaccines should give strong protection against Omicron - with the effect 'kicking in quickly' after a third dose.
He noted: “There is a view out there that in some way or another that the vaccines are no longer effective.
"Vaccines are effective in preventing your infection from becoming a serious infection - that’s their primary purpose, and that has held up.
“The thing we really want to boost is the ability of that vaccine to cut down on you spreading that infection."
With the booster campaign now ramping up, Dr Holohan advised that everyone should now do "whatever you can" to cut social contacts.
He said: “This is a difficult message to give people at Christmas time - people can feel pressure to meet up.
"But in the face of this particular infection, it really would be better if you could minimise or put off those types of social contacts."
Tánaiste Leo Varadkar today dismissed suggestions that there's 'a secret plan' to introduce more restrictions after Christmas.
Dr Holohan said NPHET can't rule out any further restrictions, but there's no concrete plan to make more recommendations at the moment.
He said: "We now think people will hear the message, and over Christmas will take measures to cut their social contacts as much as they possibly can.
“Our job is to continue to monitor what is happening. If we see things are improving, we’ll say that and that will inform the advice we give Government.
"If we see things are not improving, we have to deal with that too. But we remain optimistic.”
He said it would be a good idea for people to use antigen tests if meeting family or friends over Christmas - but stressed the rapid tests aren't a replacement for the other public health measures.