BEAUMONT Hospital's emergency department was evacuated and closed after a patient set fire to a mattress.
The incident, at one of the country's busiest A&E units, took place in the early hours of the morning.
The patient and two others barricaded themselves into a room where they also caused fire damage to curtains.
A hospital spokeswoman said the emergency department reopened after four hours.
"Shortly after 3am (yesterday) a small fire occurred in a room attached to the emergency department. Although the fire brigade was called, the fire was quickly extinguished before they arrived," she told the Herald.
"No patient or staff were injured. However, in order to minimise disruption the ED went off call from 4am to 8am. ED is now back on call," she added.
"The damage suffered was minor and the room in question is expected to be in use this afternoon," the spokeswoman said.
Dublin Fire Brigade confirmed three people blocked themselves into a room and started a fire.
It said by the time a fire tender arrived a 3.32am the flames had been extinguished and the incident was contained.
Reports stated a brigade member, who happened to have been dropping off a patient, broke the door down.
The crew member was understood to have been assaulted by one of the individuals in the room but not injured.
It is unclear whether gardai were called to the scene. The Garda Press Office said they did not have a record of the incident.
Beaumont's emergency department is one of the busiest in the country.
In the Dublin commuter area, Tallaght emergency department saw just under 73,000 patients in 2011, followed by Our Lady of Lourdes Hospital in Drogheda with just under 54,000.
Beaumont came next with more than 49,000 and the Mater Hospital had over 48,000.