The 21-year-old, who is the daughter of trainer Aidan O'Brien, was injured when her mount Druids Cross slipped two furlongs out in the second race.
She was taken away in an air ambulance with fractures to her neck, back and cheeks, according to the Racing Post.
Druids Cross, a three-year-old trained by Ana's brother Joseph, was fatally injured.
O'Brien secured her first race win in 2013 at 16 and rode for her father in this year's English and Irish Derbys.
Irish jockey Ana O'Brien has been airlifted to hospital following a fall at Killarney on Tuesday night.