The Irish Nurses and Midwives Organisation is seeking that Beaumont Hospital go off call immediately due to the unsafe conditions in its emergency department.
The Organisation says there are currently 75 patients in the department, 33 of whom are awaiting an in-patient bed.
They say this number is set to rise further throughout the evening.
INMO Industrial Relations Officer, Lorraine Monaghan said: "The hospital is in crisis since Monday yet the appropriate actions have not been taken to address the severe overcrowding levels.
"It is imperative that the hospital goes off call immediately and remains off call until such time that the situation is brought under control."
Earlier, the INMO said the level of overcrowding at Beaumont was completely unacceptable, describing it as inhumane.
Tony Fitzpatrick said staff and patients are at significant risk.
He also said there are significant problems with delayed discharges at the hospital.
The number of patients on trolleys in emergency departments, or on wards waiting for admission, has reduced to 473, down from 558 patients yesterday.
The figures were provided by the INMO.
The Health Service Executive has said it had discussions with representatives of the INMO today on the overcrowding issue.
It said it provided reassurances that measures agreed before Christmas on overcrowding will be implemented in all hospitals.
The HSE said it remained hopeful that the threatened industrial action by nurses would be called off.
Mr Fitzpatrick said the INMO has convened a meeting of strike committees from the 26 hospitals across the country with emergency departments, which will take place tomorrow.
He said notice of industrial action has been served for next Thursday and it is likely that seven hospitals will be involved.
He said that HSE management needs to be seen to be taking the problem seriously.
Management at Beaumont have appealed for people not to attend the ED due to the levels of overcrowding and said it acknowledged and regretted the difficult conditions experienced by patients at its emergency department.
The hospital said it has one of the busiest EDs in Ireland providing services to over 50,000 patients every year.
INMO General Secretary Liam Doran said while the union recommended the proposals from the Workplace Relations Commission on hospital emergency departments, its members had no confidence that they would be implemented on a 24/7 basis in a system-wide way.
He said to avoid industrial action, INMO members had to be convinced by local management that the proposals were a new beginning for emergency departments with a reduction in the number of trolleys and the addressing of staffing deficits.
"Management have got to demonstrate that those measures will be a reality - not proposals, not plans - a reality.
"And that's the challenge now coming out of the rejection vote if we are to avoid industrial action which nobody wants, but our members have had enough."
Opposition calls for additional health funding
Fianna Fáil's health spokesperson Billy Kelleher has said hospitals asking patients not to turn up at emergency departments is an indication of the level of crisis the health service is in.
Speaking on RTÉ's News at One, Mr Kelleher said additional funding needs to be allocated to the HSE in a bid to cope with overcrowding.
"The fact of the matter is, our emergency departments again are in crisis as we face into 2016. Clearly there is just not enough capacity in terms of dealing with the through-put, we don't have an additional level of nurses that were meant to be recruited over 2015 to ensure that we had enough emergency staff in place. We have a lack of emergency consultants and simply our hospital system is under huge pressure."
Mr Kelleher said the Government has failed to put a plan in place to deal with the issue of overcrowding. He added that the nurses' ballot in favour of strike action indicates their frustration with the situation.
He said a health service cannot be expected to function without adequate funding.