The INMO says elective work in Irish hospitals should be curtailed until the end of January to allow staff to deal with rising COVID-19 infections.
It comes after HSE CEO Paul Reid wrote to the country's hospital groups and said the focus should be on urgent care and COVID care over the next 14 days.
That means elective care can be "scaled down" where necessary, while private hospital capacity is also being used "to the greatest extent we possibly can", he said.
But INMO general-secretary, Phil Ni Sheaghdha, told The Pat Kenny Show more than 400 people were admitted to hospitals on Tuesday who had no bed.
"The infection rates have obviously escalated significantly, we met with the HSE yesterday.
"There is an increase in hospital admissions, as you know this time of the year is always very very busy in acute hospitals.
"Yesterday, for example, our trolley count had 420 people admitted to public hospitals for whom there were no beds.
"So our hospitals are overcrowded.
"Then when you have infection control procedures, it slows down a lot of the way we work".
Ms Ni Sheaghdha says they are looking for practical assistance from the HSE.
"We welcome the letter from the CEO to the hospital groups and the community healthcare areas.
"But we think really and truly, and based on stats we've just heard there, this is not going to be gone in two weeks.
"We believe that all of the elective work must be curtailed until the end of January to give our healthcare workers some chance of dealing with what they're facing in a safe manner".
She says this would include a push on recruitment.
"We can't have a situation where Plan B is always 'Make sure you cancel annual leave, redeploy and just use the people that are employed in all sorts of different services when we need to'.
"For example nurses have been in the vaccination clinics, have been on the testing and tracing clinics... all of the new services that this pandemic has required, they have populated.
"But their numbers have had a modest growth".
While she says intensive care is also under 'tremendous pressure'.
"We still have a lot of patients in ICU with COVID that requires us to work in a different way.
"We also have people in ICU who are, obviously, very very sick but not COVID-related at the moment.
"All-in-all it's very, very difficult".
And she says the infection rate among hospital staff has not gone down.
"The infection rate among staff continues to sit around 2.5%/2.6% of all the infections - that's very high in the context of trying to staff wards.
"And then obviously many are also close contacts."
She says about 10% of rosters cannot be filled because of COVID-related issues.