The latest figures from the Irish Nurses and Midwives Organisation (INMO) reveal 10,515 patients were left waiting on trolleys in September, making it the second-worst September on record.
University Hospital Limerick (UHL) was the most overcrowded hospital in the country, with 1,382 patients waiting on trolleys this month, followed by Cork University Hospital (1,260 patients), University Hospital Galway (1,032), Sligo University Hospital (790) and Letterkenny University Hospital (666).
The organisation warned the State is "on path to a catastrophic winter in our hospitals", adding that current conditions are unsafe for both nurses and their patients.
INMO general secretary Phil Ní Sheaghda also criticised healthcare leaders for showing no clear vision as to how overcrowding would be managed over the coming months, with the winter plan yet to be published.
"The ongoing problems with overcrowding are leaving nurses completely and utterly demoralised.
Our members are now voting with their feet and saying that they will not stand for another winter where they are demoralised, burnt out and abused in their workplace because of the excessive workloads
"We have a severe recruitment and retention problem within the health service. This week alone in a large teaching hospital in Dublin, over a dozen nurses working in a busy Emergency Department handed in their notice.
"Our members are now voting with their feet and saying that they will not stand for another winter where they are demoralised, burnt out and abused in their workplace because of the excessive workloads," Ms Ní Sheaghda said.
Commenting specifically on UHL, which is continuously the most overcrowded hospital in the State, Ms Ní Sheaghda said a recent INMO visit showed "there is no dignity" for patients, adding: "Their care is no doubt being compromised because of the conditions."
"It is not enough for the Minister for Health and senior HSE leadership to acknowledge that we are in for an undesirable winter.
"We need to know when the private hospitals will be coming on the pitch, we need to know what exact measures are being implemented to keep our nurses in the system, when extra capacity will be coming through in communities to allow discharging of patients to happen in a timely manner.
"Patients need assurances that they will be cared for in a safe environment that ensures their care is not compromised," Ms Ní Sheaghda said.