There were 760 admitted patients waiting for a hospital bed this morning, the highest number since January 2020, according to the Irish Nurses and Midwives Organisation.
INMO General Secretary Phil Ní Sheaghdha said the level of overcrowding is a danger to both patients and staff.
The hospital worst affected is University Hospital Limerick with 92 patients, with 76 waiting for a bed at University College Cork and 49 waiting at St Vincent's University Hospital in Dublin.
There were also 39 patients waiting for a bed at the three children's hospitals - 19 at Children's Health Ireland Crumlin, 15 at the National Children's Hospital, Tallaght and five at CHI Temple Street.
Today's figure equals the highest number previously reported on 6 January 2020.
Ms Ní Sheaghdha said the overcrowding situation is entirely predictable.
Speaking to RTÉ's News at One, she said the INMO had been saying since July that they were under pressure, but no new measures were introduced.
She said "misery" was the only way to describe the situation where patients are in overcrowded areas with no privacy.
"More worryingly, our members and other medical staff are saying it is not possible to guarantee safe care will be provided and that's a huge concern," she added.
Ms Ní Sheaghdha said "very high numbers" are often seen in January and interventions must be taken now.
"We know January will be worse and we know that people will have poorer outcomes," she said.
Describing the situation in some hospitals as "mayhem", Ms Ní Sheaghdha said they do not want a situation where the reputations of hospitals are damaged, because those working there are "doing their best".
"It is the environment and the policy decisions that haven't been taken and that are now delayed and need to be taken that is causing this," she said.
