The head of the HSE has said its Winter Plan is 'clearly not sufficient' to deal with 'a clear patient safety risk'.
Stephen Mulvany was speaking as public healthcare providers have been asked to move to a 7/7 operating model to deal with hospital overcrowding.
That means they have been told to work at full capacity all week, instead of just Monday to Friday.
It comes as 535 people are waiting on trolleys across Irish hospitals, according to the latest figures from the Irish Nurses and Midwives Organisation (INMO).
Mr Mulvany said the levels of flu are much higher than they had projected.
"Clearly the Winter Plan, and multi-annual plans that we've been putting in place at this stage, is not sufficient to deal with what we see as a clear patient safety risk in front of us," he said.
"That's partly because the levels of flu, in particular, are much higher than even our worst most pessimistic modelling.
"We've an exceptional season of both COVID, RSV and also flu - and unfortunately we haven't yet seen the peak of flu."
He explained how the 7/7 model would work.
"For the next two to three weekends, we have asked the hospital groups and community health organisations to move as far towards 7/7 - in other words, extra weekend working," he said.
"Our hospital services work 24/7... but they don't work at the highest capacity level, particularly on Saturdays and Sundays.
"They do most of their diagnostic work and most of their discharge work... during Monday to Friday.
"When things are running normally, they would seek to balance that out so that they get enough discharges during the week.
"Things are at the point now where we've asked the staff to come in over the next two to three weekends so that we can try and balance that load - and actually avoid, as much as possible, what could be a higher level of trolleys otherwise," he added.