Jean Kennedy Smith, the last surviving sibling of President John F Kennedy and a former ambassador to Ireland who played a key role in the peace process, has died aged 92, her daughter confirmed.
Ms Smith - who granted Sinn Fein leader Gerry Adams a controversial visa in 1994 to visit the US amid serious opposition - died at her home in Manhattan, her daughter Kym told the New York Times.
Last night, Mr Adams said her appointment as Irish ambassador had been "pivotal to the development of the peace process".
"She was a wonderful, compassionate, formidable woman who played a key role in persuading her brother Senator Ted Kennedy to support a visa for me in 1994," he said.
Though she never ran for office, she campaigned for her brothers, travelling the country for then-Senator John F Kennedy as he sought the presidency in 1960.
In 1963, she stepped in for a travelling Jacqueline Kennedy and co-hosted a state dinner for Ireland's president.
The same year, she accompanied her brother, the first Irish Catholic president, on his famous visit to Ireland.
Their great-grandfather, Patrick Kennedy, was from Dunganstown in Co Wexford.
Three decades later, she was appointed ambassador to Ireland by President Bill Clinton, who called her "as Irish as an American can be".
During her confirmation hearing, she recalled the trip with her brother, describing it as "one of the most moving experiences of my own life".
As ambassador, she helped persuade Mr Clinton to grant a visa to Mr Adams in defiance of the British Government, which branded him as a terrorist.
She later called criticism of her actions "unfortunate" and said she thought history would credit the Clinton administration with helping the peace process in Northern Ireland.
The then-Taoiseach Bertie Ahern said in 1998 that "it is not an understatement to say that if (the visa for Adams) didn't happen at the time, perhaps other events may not have fallen into place".
In 1996, though, Ms Smith had been reprimanded by Secretary of State Warren Christopher for punishing two of her officers who objected to the visa for Mr Adams.
In December 1998, Ms Smith again risked controversy by taking communion in a Protestant cathedral in Dublin, going against the bishops of her Catholic church.
Her decision was a strong personal gesture of support for Irish President Mary McAleese, a fellow Catholic who had been criticised by Irish bishops for joining in the Protestant communion service.
"Religion, after all, is about bringing people together," Ms Smith told The Irish Times.
"We all have our own way of going to God."
When she stepped down as ambassador in 1998, she received Irish citizenship for "distinguished service to the nation".
In 2009, along with her brother Ted Kennedy, she was awarded the prestigious Tipperary Peace Prize for her contribution to the Northern Ireland Peace Process.
Irish President Michael D Higgins said the death will have been received with "great sadness by many, both in the United States and in Ireland".
"As United States Ambassador to Ireland in the 1990s Ambassador Kennedy Smith played a pivotal role in the initiatives that led to the Good Friday Agreement which helped bring the violence in Northern Ireland to an end," he said.
"She will be forever remembered as the diplomat who had a sense of Irish history and of what had influenced the Irish in the United States. An activist diplomat, she was not afraid to break with convention or explore the limits of her mandate. She brought passion and clear values to her role, providing many of the elements that promoted peace and reconciliation in Northern Ireland."
Taoiseach Leo Varadkar said: "She proved an extraordinary diplomat during the crucial years leading up to the signing of the Good Friday Agreement, building lasting relationships with all sides and becoming an instrumental figure in the Northern Ireland Peace Process.
"Her courageous and determined diplomacy helped to bring peace to our island, built bridges, opened doors to all communities, and to all those striving for peace when peace was not a certainty," Mr Varadkar added.
Ms Smith was the eighth of nine children born to Joseph P and Rose Kennedy, and she tragically outlived several of them by decades.
Her siblings included the president, assassinated in 1963.