LONDON (AP) — Allies and political opponents are paying tribute to Martin McGuinness, the Irish Republican Army commander-turned-peacemaker who is stepping down as one of Northern Ireland’s most influential politicians.

British Prime Minister Theresa May said McGuinness had played a key role in moving the Irish nationalist Republican movement toward “peaceful and democratic means.”

Leading pro-U.K. Protestant politician Ian Paisley, Jr., said Friday that McGuinness’ “remarkable journey … not only saved lives, but made the lives of countless people better.”

McGuinness, from the IRA-linked Sinn Fein party, quit as Northern Ireland’s deputy first minister last week, plunging the Catholic-Protestant power-sharing government into crisis.

On Thursday he announced his retirement from front-line politics, saying he needed to concentrate on recovering from “a very serious illness.” He did not say what the illness is.