There were 11,452 patients on trolleys in Irish hospitals in October, according to new figures from the INMO.
The union says this is the worst-ever October for overcrowding and the second-worst month since records began.
They are warning that due to the overcrowding and "chronic understaffing" patients are at "grave risk".
The INMO says that according to their analysis of the latest HSE workforce figures that since 2019 there are:
The INMO claim that there are more than 1,300 nursing and midwifery vacancies in the public health sector.
"The message from the frontline is clear: patients are being put at grave risk," said INMO General Secretary Phil Ní Sheaghdha.
"There is no doubt that this situation will worsen as winter bites, unless staffing becomes a top priority for the government.
"I have written to the HSE to warn that their recruitment pause is putting lives at risk.
"We need to be encouraging more nurses and midwives into our system. Instead, we are making an already bureaucratic recruitment process even more difficult.
"In many cases, we’re slamming the door on those who want to work in our public health system."