Before her frontal lobotomy

Rose Marie Kennedy was born at her parents' home in Brookline, Massachusetts. She was the third child and first daughter of Joseph P. Kennedy, Sr. and Rose Fitzgerald. She was named after her mother, and commonly called "Rosemary" or "Rosie." During her birth, the doctor was not immediately available and the nurse ordered Rose Kennedy to keep her legs closed, forcing the baby's head to stay in the birth canal for two hours. The action resulted in a harmful loss of oxygen. As Rosemary began to grow, her parents noticed she was not reaching the basic development steps an infant or a toddler normally reach at a certain month or year...

She was deemed to have an IQ between 60 and 70... Diaries written by her in the late 1930s, and published in the 1980s, reveal a young woman whose life was filled with outings to the opera, tea dances, dress fittings, and other social interests... Placid and easygoing as a child and teenager, the maturing Rosemary Kennedy became increasingly assertive and rebellious. She was also reportedly subject to violent mood changes. Some observers have since attributed this behavior to her inability to conform to siblings who were expected to perform to high standards, as well as the hormonal surges associated with puberty...

In November 1941, when Rosemary Kennedy was 23 (and hence legally an adult), doctors told Joseph P. Kennedy that a form of psychosurgery known as a lobotomy would help calm her mood swings and stop her occasional violent outbursts. He decided that his daughter should have the lobotomy performed; however, he did not inform his wife Rose of this until after the procedure was completed. Because Rosemary had been diagnosed as mentally retarded, only his consent was necessary, not hers...

"As Dr. Watts cut, Dr. Freeman asked Rosemary some questions. For example, he asked her to recite the Lord's Prayer or sing "God Bless America" or count backwards. ... . "We made an estimate on how far to cut based on how she responded." . ... . When she began to become incoherent, they stopped."

After the lobotomy, it quickly became apparent that the procedure was not successful. Kennedy's mental capacity diminished to that of a two-year-old child. She could not walk or speak intelligibly and was incontinent... After the procedure, Rosemary was immediately institutionalized... Rosemary Kennedy died from natural causes on January 7, 2005, at the Fort Atkinson Memorial Hospital in Fort Atkinson, Wisconsin, at the age of 86.

More details at Wikipedia.  Photo (1939) via the OldSchoolCool subreddit, where there is an informed discussion.