The Emergency Department at Dublin's Mater Hospital will close for a time this Sunday to enable services to be relocated to the new state-of-the-art facility.
The department will close at 4am for 18 hours and the new facility will open at 10pm that evening.
Access to the new Emergency Department will be via pedestrian and road access on Eccles Street.
During the closure period, patients with minor injuries can attend the Mater Smithfield Rapid Injury Clinic in The Forge, beside the Smithfield Market.
For ambulances, access to the old Emergency Department will close from 10am on Sunday.
The new Emergency Department in the Whitty Building has 15 new single-patient examination cubicles for people with complex and urgent medical complaints.
It also provides for a 60% increase in capacity for resuscitating patients.
The department has a dedicated CT/X-ray suite, a 12-bay acute medical assessment unit and an ambulatory care area for managing low-impact trauma cases.
The old Emergency Department has long been the focus of controversy over overcrowding, cramped conditions and, at times, long waits by ambulance crews to get patients into the facility.
The department sees around 50,000 patients a year.
Dr Eamonn Brazil, consultant in emergency medicine at the Mater, said that the new facility will offer a vastly improved environment for patients and staff.
He said international experience is that new emergency units can expect an increase of up to 15% in patient attendances.
With the closure of the Mater Hospital's old Emergency Department, the HSE has said that other emergency departments in the Dublin area may experience an increase in patients presenting during these times.
It is encouraging the public to attend EDs for emergencies only and where possible to contact their GP, or GP Out of Hours services.
For seriously ill or injured patients, they should dial 999 or 112, or go directly to another emergency department.