Gardaí are appealing for witnesses to the accident which took place on Sunday afternoon on the Old Mallow Road.
The girl, who was visiting relatives in the Dublin Hill area, was cycling down from Old Whitechurch Road when she lost control of her bicycle and hit a wall near the bridge at Old Mallow Road.
The accident happened at approximately 2.50pm.
Emergency services were quickly on the scene and she was rushed to Cork University Hospital.
A spokesman for the Health Services Executive (South) said that the girl was in a critical condition in intensive care.
Supt Martin Shanahan said that gardaí had already interviewed some witnesses.
However, they were hoping more witnesses would come forward.
Gardaí, he said, are satisfied no vehicle was involved in the accident.
He said it was a new bicycle, but that gardaí were checking it to see if it had any defects which could have contributed to the accident.
Fr John O’Donovan, parish priest of Blackpool, said that people were saddened to hear of the accident.
“All our prayers and thoughts are with her and her family at this time,” Fr O’Donovan said.
Supt Shanahan appealed to anybody who might have witnessed the accident and not yet come forward to contact gardaí.
Anybody with any information is asked to please either telephone Watercourse Road Garda Station at 021-4558260 or Mayfield Garda Station at 021-4558510.