Beaumont Hospital in Dublin has been declared "unsafe" by the hospital chief executive Liam Duffy.
In a written warning to staff, Mr Duffy said it was "in the interest of the safety of all patients" that the hospital "regain its stability as a matter of urgency."
On the day of the warning Mr Duffy pointed out that there were 58 patients in the emergency department and one of them had been waiting 44 hours before being discharged.
His warning came just four months after the former clinical director of the hospital, Prof Shane O'Neill, resigned over patient safety concerns.
Doctors at Beaumont, one of the country's biggest hospitals, have been raising safety concerns over the past two years.
The situation has been worsened by an increasing number of patients in acute beds who are fit for discharge but are on a waiting list for nursing home beds and cannot be discharged from the hospital.
Mr Duffy, in his email, said 73 patients were currently on that list.
Prof O'Neill, in an email sent to staff before his resignation, said he could not stand over "significant clinical risks" at the hospital.
He was particularly referring to psychiatric patients in the overcrowded emergency department, which he described as "entirely unsafe and indefensible"
These were patients who were described as being suicidal or suffering from extreme psychiatric episodes and he was urging hospital management to set up a dedicated psychiatric assessment area for these patients.
Beaumont's emergency department catered for almost 52,000 patients last year and that number has increased by a further 10pc in 2014.
The hospital recorded 2,433 patients on trolleys last year.
This was the highest figure of any hospital in the country.
A statement from the hospital told the Herald that Mr Duffy's email "related to an exceptionally busy day for the hospital at the end of August."
The day was busy in both the Emergency Department and in other demands for beds but "the situation has stabilised since that day", the statement said.