Professor Sam McConkey says he believes new restrictions coming in are justified, to avoid a concurrent 'three waves at the same time'.
He was speaking as nightclubs are to shut and number limits set on pubs and restaurants from Tuesday.
Prof McConkey is head of the Department of International Health and Tropical Medicine at the Royal College of Surgeons in Ireland.
He told Newstalk Breakfast the latest Delta wave appears to be leveling off.
"The fourth wave of Delta virus, that we've been noticing in Ireland since the beginning of October, really appears now to have flattened out and leveled off.
"The number of new cases each day is pretty steady, and number of people in hospital and ICU has leveled off and is slightly decreasing.
"So we have actually, I believe, plateaued in that fourth wave of Delta.
"It will take about two or three months for those numbers to go down - so unfortunately the way out of this... takes quite a while.
"So when I model it, it's about 100 people still in hospital with Delta by the end of March".
However he says Omicron is now the new worry, coupled with other factors.
"I suppose what everyone's a little bit spooked by is the possibility of - what I would call - a concurrent three waves at the same time.
"One of the Delta one - that we're stable [sic] - two of a big Omicron wave, and three an influenza wave.
"If Omicron hadn't arrived on the scene 13 days ago, we'd be in a reasonably good place".
But Prof McConkey says he believes new restrictions from Tuesday are probably justified.
"You have to hope for the best and plan for the worst... and the worst is this triple wave."
Booster vaccine 'every one to two years'
On boosters going forward, he says it is possible they may not be needed as frequently.
"I'd be reasonably optimistic that after the third dose, many of us will get waning immunity but immunity that wane - for example - at a year or so.
"Many doses like the childhood DPT... you get three of them, and then you don't need another one until you're five and then you don't need anymore.
"So in fact as you build up more it lasts longer - so I'm expecting it'll probably be about one to two years that we'll need a new booster vaccine".
But he says we probably could have done more in the last few months.
"There was a lull in vaccines from August to October, and now in retrospect we should have been boosting the folk that were vaccinated in January/February in August/September/October.
"We can't change the past... now I think the booster campaign really is taking off and it's up to scratch.
"Not only the over-50s, but boosting everyone who's got five or six months from their vaccine".
