The Irish Nurses and Midwives' Organisation and SIPTU have expressed "grave concerns" about overcrowding and staffing levels at Tallaght Hospital in Dublin.
The INMO says its members are unable to provide adequate and safe patient care in all areas and has called for the immediate cessation of all elective surgery and the cancellation of all planned procedures.
A meeting with hospital management is to take place on Friday morning to discuss the issues.
Both unions are to meet members in the coming weeks to consider industrial action over the problems.
In a letter to management today, the INMO has asked the hospital to prioritise care for those already in the hospital and has asked for staffing numbers for nurses and healthcare assistants for 1 January this year, compared with the same time last year.
The unions have expressed particular concern regarding all ward areas, the coronary care unit, the medical assessment unit and the emergency department.
Today there were 17 patients on trolleys in the emergency department and 19 patients on trolleys on wards, awaiting admission to a bed.
Tallaght Hospital tonight said it has cancelled some non-urgent surgery and day-case procedures, due to the emergency department overcrowding pressures.
It said that cancellations are being carried out in a planned manner and patients are being notified in advance.
While this was an undesirable situation, the hospital said it was not untypical at this time of year.
The hospital said it was implementing a response plan to deal with pressures in the emergency department.
It said that trolley numbers had reduced this evening.