There are 544 patients on trolleys in hospital emergency departments, or on wards waiting for admission to a bed, according to figures compiled by the Irish Nurses and Midwives' Organisation.
The hospital worst affected is University Hospital Limerick, with 49 patients waiting.
Other hospitals badly affected are: St Vincent's University Hospital in Dublin with 47, St Luke's in Kilkenny with 36 and Naas General and University Hospital Galway, each with 35 patients waiting.
The figures exclude the number waiting on wards at St Vincent's, as the hospital is not in a position to provide this figure, according to the INMO.
Overcrowding levels have broken the 500 barrier around nine times this year so far.
The highest figure this year was 558 patients waiting on 5 January.