Almost 800 patients went without a bed at Sligo University Hospital this month according to the Irish Nurses and Midwives Organisation.
In figures published, Sligo University Hospital ranked as the third most overcrowded hospital for the month with 775 patients on trolleys.
Only University Hospital Limerick (2,174) and Cork University Hospital (1,024) had a higher number of patients.
Over 10,000 patients nationally went without a bed, with over 200 children without a bed in September.
So far this year, over 2,500 children under the age of sixteen have been treated on a trolley or chair.