The 22-year-old daughter of one of the Guildford Four, and whose grandfather was the late Robert F Kennedy, has died at the family's compound in Hyannis Port, Massachusetts.
Saoirse Kennedy Hill was at the compound, the home of her grandmother, Ethel Kennedy, 91, when emergency personnel were called yesterday afternoon to the residence.
She was taken to Cape Cod Hospital in Hyannis, the New York Times reported, citing family and friends, where she was pronounced dead.
The New York Times, and other media quoted from a statement released by the Kennedy family confirming the young woman's death, the latest in a long series of tragedies to beset one of America's leading political dynasties.
"Our hearts are shattered by the loss of our beloved Saoirse," the family statement said. "Her life was filled with hope, promise and love."
The statement quoted her grandmother as saying: "The world is a little less beautiful today."
Ms Hill was a student at Boston College, where she was a communication major and vice president of the Student Democrats, according to the New York Times.
She was the only child of Paul Hill and Courtney Kennedy Hill, who was the fifth of 11 children of Ethel and Robert Kennedy.
Mr Hill was one of four men wrongly jailed for the 1974 Guildford pub bombings, which killed four soldiers and a civilian.
They were handed life sentences, and spent 15 years in prison before their convictions were overturned in 1989, in what is considered to be one of the worst miscarriages of justice in Britain.
He married Ms Kennedy in 1993 and the couple separated in 2006.
Ethel Kennedy has endured a series of tragedies that have befallen her extended family, including the assassination of her husband, then a US senator for New York, who was shot dead in Los Angeles just after winning the California primary race for the Democratic presidential nomination in 1968.
Two of Robert and Ethel's sons also died - David, of a drug overdose in 1984 at age 28 and Michael in 1997 at age 39.
Robert Kennedy's older brother, President John F Kennedy, was himself assassinated five years earlier in Dallas, and his son, John Jr, died with his wife and sister-in-law when the plane he was piloting crashed in the Atlantic off Martha's Vineyard, not far from Hyannis Port.