The Irish Nurses and Midwives Organisation say five hospitals experienced their worst-ever overcrowding this September.
The union records the numbers of patients on trolleys every day.
It says the University Hospital and the Mercy University Hospital in Cork as well as Portiuncula, Letterkenny and University Hospital Kerry all saw higher numbers of people waiting for a bed than any other year.
A spokesperson for the INMO says overcrowding figures reached record lows in 2019 and 2020 due to the pandemic, but is warning that hospitals are "returning to the bad old days".
A number of other hospitals saw the numbers on trolleys double this month, compared with last year, including St James' in Dublin, Wexford General Hospital and Cavan General Hospital.
