Hospital overcrowding has risen to its worst level since the pandemic broke out, according to the Irish Nurses and Midwives Organisation.
The union said 376 patients were on trolleys this morning - the highest figure since 5 March 2020 before the pandemic was confirmed.
The INMO claimed the redeployment of nursing staff to vaccination duties had triggered the closure of scaling back of certain day services, resulting in additional pressure on emergency departments.
It has called for student nurses and midwives to be appointed as paid vaccinators to reduce the redeployment of nursing staff.
The union insisted that infection control and social distancing were compromised when patients were on trolleys in corridors.
The INMO said the worst-hit hospitals were University Hospital Limerick with 75 patients awaiting admission, followed by Letterkenny University Hospital (31), Cork University Hospital (30), Midland Regional Hospital Mullingar (24) and South Tipperary General Hospital (23).
It called for an urgent national intervention - particularly in UHL - along with a strategy to reduce the volume of staff being redeployed for vaccinations.
INMO President Karen McGowan said that although Covid levels were reducing, the long-standing trolley crisis was again "rearing its head".
"Our members are seriously concerned that we will swing from the Covid crisis back into an overcrowding crisis. They need to know that the HSE will not tolerate overcrowding and ensure that safe staffing levels are implemented," she stated.
The union's General Secretary Phil Ní Sheaghdha warned the Health Service Executive not to allow trolley figures to rise and rise.
"We have kept trolley figures suppressed for much of the pandemic, but we are slipping back into old bad habits," she said.
"Overcrowding is simply unsafe for patients especially during a pandemic. It is placing intolerable pressure on an exhausted workforce, who are now working to provide mass vaccinations in addition to a Covid and non-Covid healthcare service."
Responding to INMO claims regarding hospital overcrowding, the Health Service Executive said the health service had continued to deliver emergency care while also mitigating the demands associated with the Covid-19 pandemic.
It said Emergency Department activity levels and hospital admissions are now equal to or higher than the levels seen in 2019.
It stressed that many patients in Emergency Departments and admitted to acute hospitals have "complicated illnesses, complex care-needs and require longer stays in hospital".
The HSE said the average length of a hospital stay has extended by one day, meaning more bed days and care needs are required to support the same number of patients.
"Despite these challenges, patients are waiting less time in the ED to be discharged or admitted to hospital than in 2019 and in line with the significant investment in capacity arising from the HSE's Winter Plan, ED Trolleys are significantly less than in 2019," the spokesperson said.
She added that the HSE could reassure the public that all patients attending an Emergency Department will be seen and, as always, the sickest patients and those requiring urgent treatment and care will be prioritised and seen first - with separate pathways for those with possible Covid-19.
She also pointed to other options for accessing urgent care including GPs, pharmacists, out of hours services and minor injury units.
A spokesperson for the Department of Health confirmed that a statutory instrument will be signed off later this week permitting students above first-year level from a number of professions to be recruited as vaccinators.
Those eligible to apply will include those studying medicine, nursing, midwifery, dentistry, physiotherapy, pharmacy and optometry.
The number of student/vaccinators to be recruited will depend on the level of need in a particular location - and will be a matter for the HSE.
Many of the posts will be short-term, effectively for the summer months, which may suit students.
The spokesperson could not confirm the salary level that will apply.