University Hospital Galway was the third most overcrowded hospital in the country during the month of October, according to new figures from the INMO.
Over the past month, 885 patients were on trolleys at the Galway hospital - with only the University Hospitals in Limerick and Cork experiencing higher levels of overcrowding.
The new report shows there were not enough hospital beds for almost 11 and a half thousand patients in Irish hospitals this month - and says it's the worst-ever October for overcrowding.
The INMO has warned that patients are at grave risk, due to overcrowding and chronic understaffing.
INMO General Secretary Phil Ní Sheaghdha says this situation will worsen as winter bites, unless staffing becomes a top priority for the Government.
She has also written to the HSE to warn that their recruitment pause is putting lives at risk.
At Mayo University Hospital, there were 261 patients on trolleys during the month of October - up from 166 in October 2018.
Sligo University Hospital also saw an increase in the number of patients on trolleys - from 716 in October last year to 885 this month.