9,439 hospital patients were left waiting for beds over the past month, the highest ever number recorded in July.
The figures are 33% higher than the same month last year, according to the Irish Nurses and Midwives Organisation (INMO).
Among the 9,439 patients were 45 children.
University Hospital Limerick recorded the highest emergency department waiting figures with 1,293 people left on trolleys since the beginning of July.
Cork University Hospital had the second-highest number with 1,079 patients.
INMO Director of Industrial Relations, Tony Fitzpatrick, says the overcrowding is being driven by understaffed facilities and a lack of beds.
"Currently over 700 patients cannot be discharged from hospital," said Mr Fitzpatrick.
"In the meantime, hundreds of frontline nursing and midwifery posts are currently vacant due to the HSE’s dysfunctional and bureaucratic employment control processes.
"Vital roles across all services, at all grades, in all hospitals are left unfilled. This has direct negative consequences for patients.
"We expect increased demands on the health service in winter, but now even summer sees patients crammed into corridors on trolleys.
"It is creating unacceptable risks for patients and health workers alike."