The chief clinical officer with the HSE says more than half of those in ICU are unvaccinated against COVID-19.
Dr Colm Henry was speaking as there were 941 patients with COVID in hospital on Thursday - with 90 in intensive care.
He told Newstalk Breakfast: "I know people will say it's unfair to focus [on these numbers] but these are facts, these aren't opinions.
"These are facts that 5% of the eligible population are unvaccinated, and yet over 50% of those in ICU are unvaccinated".
And he says the current wave of Omicron is a having a huge impact on hospital staffing levels.
"We're seeing this huge surge in cases right across Europe, right across the world - record numbers being beaten each day.
"And we're seeing this play through not in so much severe way through the hospitals, but... one of the big disruptive effects of this is the amount of staff being identified because of the power of this particular variant to breakthrough.
"That means we're seeing more staff get infected, not nearly as many people get seriously ill, but more staff get infected or be designated as close contacts.
"And that in itself is having a highly disruptive effect on healthcare delivery".
But he says staff who are close contacts, and not symptomatic, could be called back to work.
"There's a couple of elements of existing policy we're trying to invoke in order to protect services as much as we can - including what we call derogation.
"[This is] allowing us for asymptomatic contacts for healthcare workers to come back to work earlier - provided of course they're asymptomatic, have a negative antigen test and adhere to those infection prevention control measures.
"Without that, we would be be in severe trouble - there's no doubt about it".
Dr Henry says positivity rates from Omicron are high, but the vaccines protect against severe illness.
"The positivity rates are just mind-boggling at this stage: 60% positivity in community testing.
"And then the transmission within households - 35, 40% - very high.
"There's always hope that existing vaccination programmes do protect against severe illness - that's something we didn't have at the beginning."
And he says they should know more about those in hospital with COVID, as opposed to because of COVID, "by next week".
"We'll have a much better position to characterise not just the patients in there at the moment, but those coming in too".