The Irish Nurses Organisation (INO) said today it had been informed that 32 beds at James Connolly Memorial Hospital in Blanchardstown are to close.
The INO said the move would have "devastating consequences for patients, staff and the local community".
The union said that closure will affect hospital admissions through the Accident and Emergency department.
Speaking today, INO Industrial Relations Officer, Ms Mary Fogarty said: "This level of reduction cannot be sustained given the demands placed on this hospital via the Accident & Emergency department."
"In tandem with this are the current closures of 84 beds at the Mater and the possible closure of another 100 beds in the Mater and Beaumont."
"The health service is now on its knees and those who make these decisions have no insight into the consequences for patients, families and staff on the ground," she said.