Overcrowding in hospital emergency departments and wards has increased by 51%, when compared with the same month last year.
According to the Irish Nurses and Midwives Organisation, there were 7,775 people being treated on trolleys in June - a record high for the month.
The organisation has said this is the 12th month in a row where overcrowding has been higher than the same period in the previous year.
The INMO began its 'Trolley Watch' 12 years ago.
It says the highest annual increase in the number of patients on trolleys was recorded at St Vincent's Hospital in Dublin, where overcrowding increased by 332% year on year.
Outside Dublin, the highest overcrowding rates were recorded at University Hospital Waterford, which was 178% higher than last year.