Some staff at St Vincent's Hospital emergency department in Dublin are to ballot on industrial action.
INMO members say they are concerned over patient safety and overcrowding in the department, and they may vote for strike action.
In a statement, the INMO said: "Nurses took the decision to ballot...following months of overcrowding inevitably leading to increased risk to patients and intolerable conditions for nurses."
They claim hospital management is refusing to accurately report the number of patients who are treated on trolleys.
INMO industrial relations officer Philip McAnenly said: "There has been chronic and intolerable overcrowding in the emergency department in St Vincent's Hospital for a number of months and the nurses have tried to raise concerns with the management of the hospital without success.
"In recent times, the nurses have discovered that the incorrect figures - they believe - are being returned to the INMO's trolley watch (recording service) that the Minister for Health Leo Varadkar depends on, on a daily basis."
According to the INMO’s trolley watch statistics, 3,331 patients spent time on a trolley in the hospital's emergency department, awaiting an in-patient bed, from January to August this year - an increase of 137% on the same period in 2014.
The INMO staff in the hospital say the manner in which management in St Vincent’s report the number of patients on trolleys underestimates the level of overcrowding each morning, because management in the hospital "have refused to count trolleys in the same way as other hospitals".