Fourteen children had to be evacuated from the paediatric unit of Limerick University Hospital after a fire broke out in the building.
The waiting-room of the emergency department was also emptied as smoke filled parts of the hospital some time about 6.30pm.
The Health Service Executive (HSE) believes the smoke came from a fire in the back-up power supply, located in the plant room on the roof of the hospital labs.
"A fair amount of smoke entered the hospital buildings so we relocated 14 patients from the paediatric unit and moved them elsewhere," a spokesman for the HSE said.
"There was a smell of burning and a volume of smoke but we can confirm that no-one was injured and the running of the hospital is getting back to normal."
A number of fire brigade units attended the scene while hospital staff opened doors and windows, and turned on fans to get rid of the smoke.
Fire chiefs gave staff the all-clear to return to work as usual.
A technical team is expected to examine the plant room to determine exactly how the fire began.