Figures from the Irish Nurses and Midwives Organisation show that 685 people were without a bed at the hospital.
The union says 4,499 people were waiting for beds last month- more than 9 times higher the April figure.
INMO general secretary Phil Ní Sheaghdha says they're concerned about how the situation will play out for the rest of the year and that staffing levels must be prioritised to ensure the safety of patients and staff.
