University Hospital Limerick was the third busiest hospital in Ireland last month.
Figures released by the INMO also show that overcrowding at the hospital has worsened when compared with last year, but not by as much as in other areas.
The Irish Nurses and Midwives Organisation say over 7,700 people were on trolleys in Irish hospitals last month.
538 people were on trollies or in overflow areas of wards at University Hospital Limerick.
This is a 7% increase on the same month last year, but the rate of increase is much less than the national average.
The figure is also more than 200 less than in May of 2013.
There has been a renewed focus on trolleys at UHL in recent days, in light of the case of a 101 year old Clare woman who spent 25 hours waiting to be admitted there.
The INMO says her treatment, and that of another woman in a similar case at Tallaght, is a damning indictment of our society
