10,538 patients, including 273 children, went without a bed in hospitals nationwide in October.
The INMO has described the high levels of overcrowding as "concerning" and adds there's "acute pressure along the western seaboard."
557 patients are on trolleys in hospitals around the country today, and the INMO says the system needs to get a grip on the out of control levels of overcrowding that occur following public holidays.
It has called on both the HSE and the Government to "outline what steps are being taken both at national and local level to dramatically reduce the number of patients on trolleys."
The top five most overcrowded hospitals are University Hospital Limerick, Cork University Hospital, Sligo University Hospital, University Hospital Galway, and The Mater in Dublin.