The Irish Nurses and Midwives Organisation have welcomed extra funds but said the HSE's winter plan will not work without extra staff.
Earlier today, the HSE unveiled a €26m winter plan to tackling hospital overcrowding.
The Executive said it has learned lessons from last winter and is set to provide extra beds, home care, extra medical resources, increased nursing, pharmacy and laboratory staff and therapies support.
The INMO has responded to the plan by saying it will not have an impact without extra staff.
They said in a statement that "the ongoing recruitment pause meant that it would be impossible to staff any additional services."
The statement added: "The HSE’s plan predicts that nearly 4% more patients will attend emergency departments this winter compared with last.
"Yet despite this projected growth, HSE figures show that there are 400 fewer staff nurses and midwives than December last year working in the HSE."
Phil Ní Sheaghdha, INMO General Secretary, said it is a case "of too little, too late."
She said: "The winter plan provides some welcome extra funding, but it is impossible to expand services with a contracting workforce.
"Even setting aside winter, the health service is severely understaffed.
"That’s leading to dangerous workplaces for our members, and unsafe conditions for our patients.
The annual winter surge is entirely predictable, yet once again we are scrambling to plug the gaps in mid-November. It simply isn’t good enough.
"The HSE need to drop their recruitment pause and ensure safe staffing levels across the health service. The only hope of a safe winter is a properly staffed health service."
Speaking earlier, HSE chief operations officer Anne O’Connor said the HSE is focused on ensuring that patients are provided with the appropriate care to meet their needs as quickly as possible.
"Building on the lessons learned from last winter, we will have a number of initiatives with a particular focus on the timely discharge of patients from hospital to appropriate care in the community including home care, step-down/transitional care or long-term care," she said.
"All our Hospital Groups and Community Health Organisations have joint Winter Plans in place to prepare for and to manage service pressures in their areas this winter, which we have been planning for since July.
"Acute hospitals are continuing to see a year-on-year increase in the number of patients requiring treatment and care.
"By the end of October, 1.1 million patients had attended our 29 emergency departments, 3% more than at the same time last year."
Green Party health spokesman Dr Seamus McMenamin criticised the HSE Winter Plan and said it does nothing to address the structural problems in the HSE.
“Structural problems in the HSE lead to overcrowding in our hospitals and is at this point like moving the deckchairs on the Titanic,” he said.
“The shortage of GPs and hospital consultants means that ‘senior decision makers’ will not be available in many instances and this can not be addressed by December and there does not appear to be any will among the Government to address this in the medium to long term."