LONDON — Seeking to salve an old wound at a time of sorrow for Britain’s royal family, the political leader of the Irish republican movement apologized on Sunday for the 1979 assassination of Louis Mountbatten, an uncle of Prince Philip.
Mary Lou McDonald, the leader of Sinn Fein, which was once the political wing of the underground Irish Republican Army, told a London radio station, “Of course, I am sorry that happened; of course, that is heartbreaking.”
Ms. McDonald offered the landmark apology a day after Queen Elizabeth II buried her husband, Philip, in a ceremony at Windsor Castle that paid tribute to his military career. His uncle Lord Mountbatten, a celebrated commander during World War II who later served as the last viceroy of India, overseeing its partition and transition to independence, was killed after a bomb exploded on his fishing boat off the coast of Ireland.
The assassination, carried out by members of the I.R.A., was one of the highest-profile attacks during the Northern Ireland Troubles, and the one that struck closest to the heart of the royal family. In addition to his ties to Philip, Lord Mountbatten was friendly with the queen and a mentor to the couple’s eldest son, Prince Charles.
Speaking to Times Radio, Ms. McDonald said Sinn Fein was now engaged in peaceful politics. Asked if she would apologize to Charles for the assassination, she said that British armed forces had carried out violent acts in Northern Ireland but added that she regretted the killing of Lord Mountbatten.
“It is all our jobs to ensure no other child, no other family, no matter who they are, suffers the same trauma and heartbreak that was all too common on all sides of this island and beyond,” Ms. McDonald said. “I am happy to reiterate that on the weekend that your queen buried her beloved husband.”
Lord Mountbatten was vacationing at his Irish home, Classiebawn Castle, in August 1979 when the I.R.A. placed a radio-controlled bomb on his boat, detonating it when he was several hundred yards offshore, lobster potting with members of his family.
In addition to Lord Mountbatten, the explosion killed Lady Brabourne, the mother-in-law of his elder daughter, Patricia; his 14-year-old grandson, Nicholas Knatchbull; and a friend, Paul Maxwell.
The I.R.A. quickly claimed responsibility for the attack, characterizing Lord Mountbatten, who had once led Britain’s armed forces, as a legitimate target in its armed struggle against the British government. Gerry Adams, then a top official in Sinn Fein, said at the time, “What the I.R.A. did to him is what Mountbatten had been doing all his life to other people.”
Mr. Adams later softened his position, without giving it up. After a meeting with Charles in 2015, he said both men had expressed regret for the bloodshed as the Troubles escalated after 1968. But he repeated his contention that Lord Mountbatten knew the risks of traveling to Ireland during that time.
“I stand over what I said then,” Mr. Adams said. “I’m not one of those people that engages in revisionism. Thankfully the war is over.”
Sinn Fein is historically identified by its links to the Irish Republican Army, a legacy it has never been able to shake with older Irish people. But under Ms. McDonald’s leadership, the party has successfully appealed to younger urban voters with policies to deal with Ireland’s acute housing crisis.
In elections last year, Sinn Fein won roughly the same number of seats in Parliament as each of Ireland’s two establishment parties, Fianna Fail and Fine Gael. Since neither wanted to form a government with Sinn Fein — in part because of its I.R.A. legacy — they agreed to enter into a formal governing coalition for the first time.
A senior diplomat noted that Sinn Fein’s gesture brought it in line with the rest of Ireland, where the killing of Lord Mountbatten has long been condemned.
“Sinn Fein’s apology, even if belated, is a welcome step in helping to leave the past behind and build a better future on the island,” said Bobby McDonagh, a former Irish ambassador to Britain.