More than 100,000 people have been forced to wait on trolleys and chairs for beds this year
November saw the worst hospital overcrowding on record according to the INMO.
The nursing union said over 9,500 patients were forced to wait on trolleys and chairs for beds last month.
It said the figures mark an 11% increase on last year and is more than twice as bad as 2006, when it first began recording the figures.
It said 2018 as a whole is already the worst year on record for overcrowding – with the amount of patients who have been left waiting for beds this year passing the 100,000 mark earlier this week.
INMO General Secretary Phil Ní Sheaghdha said much of the crisis is down to understaffing as a result of low pay levels in the health service.
“Behind each number is a vulnerable patient suffering in poor conditions, with overworked staff pulling out all the stops to provide the best possible care,” she said.
“2018 is already the worst year on record for patients on trolleys and there are still several weeks to go.
"January and February are typically the worst months, so nurses and midwives will be looking to the New Year with a sense of dread.
“But this isn't just a winter problem anymore, it's a year-round problem.
“The health service needs more beds. Extra beds require extra nurses, but the HSE simply can't hire enough on these wage levels.”
The nursing union said the health service now has 227 fewer staff nurses than December 2017.