The undersized girl – whose nationality is unknown – suffered a cardiac arrest on an Air Canada send off from Toronto, Canada, to the British capital.
The plane – flight AC-868 – was sidetracked to Shannon Airport in Ireland.
It landed at 7.40pm and the girl was rushed to Limerick Sanatorium.
Doctors battled to save the youngster – but sadly she died.
She is believed to experience been travelling with three adults when she was taken ill.
The Boeing 787-800 jet was west of County Mayo, in northwest Ireland, when the gang advised air traffic controllers that they needed to divert and land as ultimately as possible.
The pilot chose to head to Shannon Airport, in the west of Ireland.
The group asked medics to be standing by for their arrival.
An ambulance and rapid answer advanced ramedic unit from Ennis were sent to the airport onwards of the flight’s arrival at 7.40pm.
The county coroner has been informed of the extir tion while a post mortem examination will be arranged.
The flight withed its journey to London at 9.20pm.