The HSE has appealed to patients not to attend Letterkenny University Hospital unless they require emergency care.
It follows a massive spike in the number of people coming to the A&E department in the past day.
A hospital spokesman said a total of 155 patients have presented at the hospital looking for treatment in the past 24 hours alone.
Many of these patients remain in the Emergency Department awaiting a bed, said the hospital spokesperson.
A statement from the HSE said “The Emergency Department at Letterkenny University Hospital is extremely busy this week and the hospital has admitted a significant number of ill patients, many of whom remain in the ED, awaiting a bed.
“In the past 24 hours, 155 patients attended the Emergency Department. This is significantly higher than the average number which ranges between 105 and 115 patients per day.
“Hospital management regrets the inconvenience and distress the long delays cause patients and their families. The hospital prioritises those in most need, this unfortunately may lead to delays for less urgent patients.”
The statements asked patients who do not require emergency department care to contact their GP or GP out of hours service in the first instance.