There are 75 patients on trolleys waiting for a bed in Tallaght Hospital's accident and emergency unit, according to the Irish Nurses' Organisation (INO).
A number of patients have spent over eight days on a trolley, and the hospital has been forced to close its day ward to cater for the overspill from A&E, the INO claimed.
A spokesman described the situation at the A&E unit as "chronic" and "fit to burst".
A spokeswoman for Tallaght Hospital said the authorities were due to hold a meeting on the situation today and that the hospital would not be commenting until after.
The INO has called on the Health Service Executive to explain why the situation in Tallaght was so much worse than any other hospital in Dublin.
According to figures compiled by the INO, a total of 392 patients are today lying on trolleys awaiting admission to wards in the Republic's hospitals.
But Tallaght Hospital has nearly three times more patients on trolleys than any other hospital.
The figures show that 75 patients lay on trolleys in Tallaght Hospital this morning, with 28 at St James's, 25 in Beaumount and 23 in the Mater.
Elsewhere, 26 patients were on trolleys in Cavan General Hospital while the figures were also high at Letterkenny Hospital (25), and Mayo (21).