There was an 8% increase in hospital overcrowding last month, compared to July 2013.
Trolley Watch figures from the INMO show the problem was worst in emergency departments, but some wards were also affected.
The Irish Nurses and Midwives Organisation is calling for more funding to allow beds to be re-opened and extra staff to be recruited.
INMO General Secretary Liam Doran said July was usually a quieter month in hospitals, but that had not been true this year.
"We have reduced bed capacity (after) 2,000 beds closed across the country because of budgetary cutbacks," he said. "So, we have increased demand for admission and an inability to provide beds for those patients.
"Normally you would look at the summer period as bringing a slight alleviation of pressure. There has actually been an increase in pressure and that's why we have a marked increase in overcrowding in some hospitals during July."