DUBLIN — Gerry Adams, a pivotal figure in the political life of Ireland for almost 50 years, said Saturday that he would step down as leader of Sinn Fein, the main Irish Republican party, after more than three decades.

Reviled by many as the face of the Irish Republican Army during its campaign against British rule in Northern Ireland, Mr. Adams reinvented himself as a peacemaker in the troubled region and then as a populist opposition member of the Irish Parliament.

At a packed party conference in Dublin, Mr. Adams said that he would be replaced as its president at its next annual gathering and that he would not run for re-election to Parliament.

“Leadership means knowing when it is time for change,” he said in an emotional speech. “That time is now.”