Three Munster-based acute public hospitals were worst affected by overcrowding in May, it has emerged.
University Hospital Limerick had the highest number with 1,102 admitted patients on trolleys waiting for a hospital bed.
Cork University Hospital had 824 patients waiting and South Tipperary General Hospital had 661.
Throughout the month 9,015 admitted patients were on trolleys, according to the latest monthly analysis by the Irish Nurses and Midwives Organisation.
Among the patients having to wait for a bed were 78 children.
The number of trolley waiters has increased by 114% since May 2006 when the INMO started compiling the figures.
INMO general secretary, Phil Ní Sheaghdha, said the latest monthly figure is a cause for concern.
“We are entering the milder summer months when predictable seasonal illnesses are lower yet the level of overcrowding is worsening,” she said.
Ms Ní Sheaghdha said overcrowding in May this year is now at the same level as January five years ago.
“Clearly this is a capacity deficit and requires immediate investment in additional hospital beds,” she said.
Meanwhile, around 500 ambulance personnel who are members of the Psychiatric Nurses Association are staging a 24-hour strike over the right to be represented by the PNA as their union of choice.
The strike involves paramedics, advanced paramedics and emergency medical technicians. The union has warned of more 24-hour strikes in the coming weeks.
The National Ambulance Services (NAS) was managing the situation despite the challenges, the HSE said.
Managers who are qualified paramedics were carrying out front-line duties and the Department of Defence made a number of crewed ambulances available.
The HSE said ambulance personnel are already well represented by three trade unions.
"Recognising break-away unions has a destabilising effect on good industrial relations," the health authority said.