Keenan, 66, joined the IRA around 1970 and eventually became quartermaster of the Belfast brigade and involved in Belfast bombings, The Times of London reported Wednesday.
Keenan, who spent 19 years in prison for his IRA activities, became known as the right-hand man for the imprisoned Sinn Fein President Gerry Adams.
As the peace process began in the late 1990s, Keenan supported the dual strategy of engaging in peace negotiations while threatening to return to violence if demands weren't met, the British newspaper said.
In 2001, he called politics and violence "legitimate forms of revolution," that "have to be prosecuted to the utmost."
Keenan was the IRA's go-between with the Independent International Commission on Decommissioning. His role in negotiations prompted Adams to say, "There wouldn't be a peace process if it wasn't for Brian Keenan."
Adams said Keenan's death was a shock for republicans.
"Brian was a formidable republican leader over 40 years of activism," he said. "He was a man of tremendous energy, even in the face of a debilitating illness."