The figure is up significantly on the same period last year
Some 542 patients were left waiting on trolleys in Naas General Hospital in January.
That's 167 patients more compared to the same period last year, according to the INMO's trolley watch figures
11,289 people were recorded on hospital trolleys nationally since January 1st, that's up 2, 500 on the same time last year.
University Hospital Limerick continues to be the worst affected by over-crowding, accounting for 10% of the overall figure, at 1,180.
Cork, Galway, Letterkenny, and St. Vincent's in Dublin make up the top five - totalling of over 3,000.
That's in stark contrast with the bottom five - Tullamore, Portlaoise, the National Children's Hospital, Connolly Hospital, and Waterford - with a combined total of 213.
Naas resident and INMO General Secretary Phil Ní Sheaghdha commented:“A lack of adequate planning has put unnecessary stress on nurses and the patients they are trying to provide care for throughout the month of January.
"The number of patients on trolleys in wards outside our emergency departments have been unacceptably high. This practice should not be allowed to continue as a measure to try take pressure off our emergency departments.
“According to two recent opinion polls, over 75% of people would not visit an Emergency Department.
"The strategy of telling people to not attend hospital will have knock-on effects across the health service for months to come.
"We cannot continue to accept the wait until things get unbearably bad approach before an attempt is made to lessen the pressure on our public hospital system.
"Nurses and their patients deserve better than this," she added.