DUBLIN — Seamus Mallon, a dogged champion of nonviolent struggle and one of the key architects of peace in Northern Ireland, died on Friday at his home in Markethill, County Armagh. He was 83.

His death was confirmed by his family, who said he had been treated for cancer.

As a leading member of the Social Democratic and Labour Party, which promotes Irish unity through nonviolent means, Mr. Mallon spent much of his life campaigning for an end to killings and abuses by all sides in the conflict in Northern Ireland known as the Troubles — including the republican gunmen from his own Roman Catholic community, who were fighting to unite north and south, and the British forces and Protestant loyalist gangs that sought to defend the union with Britain. At least 3,500 people died in 30 years of bombings and shootings.

The leader of his party, John Hume, shared the Nobel Peace Prize with the unionist leader David Trimble for their roles in the Good Friday Agreement of 1998, which ended the conflict. But it was Mr. Mallon, a stickler for both principle and detail, who worked out the nuts and bolts of the party’s position, and who held the party together during tense, drawn-out talks.

He then became the party’s leader, and Mr. Trimble’s deputy first minister, in the first power-sharing administration set up under the deal. He endured three years of often prickly collaboration with Mr. Trimble before resigning to care for his ailing wife, Gertrude Mallon.