The number of people in intensive care units will "hold if not grow" in the coming days, the health service chief has said.
Paul Reid said ICU admissions are a "real challenge" with delayed discharges from units and an increased number of deaths.
He told RTÉ's This Week that something has changed in terms of the level of transmission and the impact of the virus.
A total of 1,931 people were in hospital with the virus on Sunday afternoon including 218 in ICU.
Twenty-four beds were available in intensive care but eight hospitals had reached surge capacity.
"Talking to the ICU consultants I think there's a huge learning in the last three weeks about what we're seeing," Mr Reid said.
"There are still 430 patients who are receiving critical care outside of ICU with advanced respiratory support."
Mr Reid said a third of those patients could end up in intensive care units or die.
"There's a very significant pipeline, unfortunately, we would see continuing into ICU. I'd expect to see ICU numbers holding if not growing in the coming days," he said. "That's a real challenge for us."
Asked about the Irish National Nurses and Midwives Organisation's concerns over a lack of high-grade face masks for nurses, Mr Reid said there was a "good supply" of up to 1.3 million masks per week for healthcare staff who required them.
He also said officials have deployed antigen testing recently to assess where there are outbreaks in hospitals, and have secured another 500,000 antigen tests. He said this will be brought into the hospital system from Wednesday.
Mr Reid also said officials are "very anxious" to get back to testing close contacts.
"We do aim to get back to it," he said. "We would need (cases) to be sustained below 2,000 for a few days for that to happen but we're very anxious for that to happen."
Asked about the announcement by AstraZeneca of possible delays in the production of its vaccine, Mr Reid said he believed shortfalls will be a "feature of the supply" and setbacks will happen.
He said the HSE is planning on rolling out the vaccine for over-70s as envisioned, having been assured by AstraZeneca that it would receive its February delivery as planned.
"So our plan to commence the over-70s in the middle of February will continue," he said. "We're not quite sure of the impacts beyond that.
"I do know that in a similar way when we had the same announcement from Pfizer it had triggered very significant concerns across the EU and from the Taoiseach, and they do expect, as they've said already, very strong conversations to happen to assess the health impacts."
The Taoiseach described the AstraZeneca production issues on Saturday as a "real problem" and warned it could delay the rollout of the vaccine to over-70s.
On Saturday Health Minister Stephen Donnelly described the news from AstraZeneca, which has advised the EU of a reduction in vaccine supply in February and March, as a "real setback".
"The numbers are still tentative and AstraZeneca is due to provide more exact figures at a meeting early next week," he said. "Will provide an update as soon as possible."
Sunday saw a further 23 deaths and 1,378 new cases of Covid-19 confirmed by the Department of Health. All the deaths occurred this month.
The median age of those who died was 84 and the age range was 61-99.
It is the second day in a row since the start of the year that the number of new cases has fallen below 2,000.