Ten ambulances were left queueing at Letterkenny University Hospital (LUH) on Tuesday due to high levels of overcrowding.
The Irish Nurses and Midwives Organisation (INMO) has called for an urgent meeting with the hospital's senior management to discuss what the group claims is "dangerous overcrowding".
The INMO said it has informed all staff that the hospital is nearing major incident level capacity.
The group's industrial relations officer, Neal Donohue said the level of overcrowding in Letterkenny is "extremely concerning", adding: "The Emergency Department (ED) is currently at full capacity with nurses struggling with overflows of patients who have been admitted.
"All staff who can be available to work in the emergency department have been asked to attend."
INMO members have said there is no available space for trolleys, with the hospital reaching out to nursing homes asking them to take patients if possible.
"Staff, patients and the people of Donegal deserve better," Mr Donohue said. "The INMO has been long sounding the alarm of the problems in LUH with hospital management, the Saolta Group and the HSE."
The calls come as trolley-watch figures from the INMO show 48 people were waiting for a bed in LUH on Wednesday morning - 17 in the ED and 31 in wards elsewhere in the hospital.
Overall, 521 patients were waiting for a hospital bed around the country this morning, 389 of whom were waiting on a trolley in an ED, 15 of these being under 16.
University Hospital Limerick (UHL) was once again the most overcrowded hospital in the country, with 81 patients on trolleys.
