The Irish Nurses and Midwives Organisation has said their industrial action at St Vincent's Hospital in Dublin will continue until overcrowding is addressed.
Nurses at the Emergency Department are engaging in a work to rule, meaning only nursing duties are carried out.
The INMO has said patient safety will not be affected, and that emergency phone calls will be answered.
Director General of the HSE Tony O'Brien has said hospital management have to engage with staff: “In each individual hospital there are local circumstances and we are looking to the individual hospital management to address those in a constructive way with their own staff," Mr O'Brien said.
“In this case St Vincent’s Hospital is a voluntary hospital, I was there myself yesterday, and I know that they are seeking to engage as meaningfully as possible to improve conditions for both patients and staff.”