Professor Philip Nolan has said we should see signs if Level Five is working in around three weeks time.
The NPHET member urged people not to get bogged down by daily case figures for the next couple of weeks, outlining that they will fluctuate for a period.
He also said that until a vaccine is found, everyone will have to socialise in different ways than we're used to.
Asked about when we may see signs that new restrictions, which begin at midnight, are having a positive effect, he said: "I lot depends on how people have been behaving over the last week or 10 days.
"People realise there's a problem, people will have pulled back. If that's happened, we might see cases top out sooner, within a week.
"On the other hand if people have been mixing as normal or even accelerating their mixing in anticipation of these restrictions, it could be two weeks before cases top out, at perhaps several hundred more than we're seeing now, and then begin to decline...
"I think we need to stick with this, again to think back to where we were in April and May, we were very careful as a population...we need to go back there.
"We should see signs in three weeks."
When questioned about when we can see a change in the number of deaths, he told RTE's Morning Ireland: "Sadly, the fatalities take longer, it's typically nine to 14 days between notifying a case and somebody dying, and it could be longer. It can be up to a month.
"We did see a number of deaths reported yesterday that we didn't see for a long time and we may see those kinds of numbers cropping up again over the next couple of weeks. That's one of the drivers for these restrictions."
