He's been vilified as a terrorist, hailed as a peacemaker and long considered one of the defining figures in Northern Ireland's troubled history. And now after 35 years in the limelight, Gerry Adams is stepping aside and giving up the leadership of the republican movement at a critical juncture.

Mr. Adams, 69, will formally resign as the national leader of Sinn Fein on Saturday, ending a remarkable career that began as a barman serving drinks in a Belfast pub and saw him become one of the best known politicians in Northern Ireland, feted by presidents, prime ministers and royalty. Along the way he's been shot five times, had his house blown up and built Sinn Fein into a political force encompassing Northern Ireland and Ireland. All while being equally loved and loathed by millions.

His departure marks a radical transition for the party and a huge challenge for new leader Mary Lou McDonald, a long-time deputy leader of Sinn Fein and a member of the Irish parliament, or Dail. And yet there are many within the party who will quietly celebrate Mr. Adams's resignation, believing he has become too divisive and outdated, and that Sinn Fein needs to move away from its roots in the militant Irish Republican Army to broaden its appeal. The party's popularity has largely stalled and none of the other major parties in Ireland will form a coalition with Sinn Fein because of Mr. Adams's IRA baggage.

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Mr. Adams has always denied being a member of the IRA, but under his leadership Sinn Fein has been seen as the IRA's political arm and the party has drawn sharp criticism for being terrorist sympathizers. "I will never disassociate myself from the IRA," he said last year in announcing his retirement. "I've condemned the IRA on many occasions and I particularly regret the fact that ordinary people – citizens, civilians – were killed or injured at the hands of the IRA."

Mr. Adams won accolades for bringing the IRA into talks that led to the 1998 Good Friday Agreement, ending decades of sectarian violence known as the Troubles that killed more than 3,500 people. But in recent years he has become a symbol of the past and has run into trouble for failing to report allegations of sexual assault by his brother and confronting new questions about his role in the murder of an IRA informant in 1972. Many Sinn Fein insiders see Ms. McDonald, 48, as the face of a new generation that has little interest in the old battles of the Troubles. Unlike Mr. Adams, who grew up in a working-class Belfast family steeped in IRA loyalty, Ms. McDonald hails from a middle-class part of Dublin and has no ties to the IRA.

"His retirement will be a really, really radical change for Sinn Fein because he has been the leader for over 30 years and he's the one defining celebrity of the party," said Malachi O'Doherty, a Belfast-based journalist who has written a biography of Mr. Adams. "If what we are seeing is the beginning of a progressive evolution away from their militaristic past, that would be quite remarkable."

There's no doubt that Mr. Adams leaves big shoes to fill and that Sinn Fein will be hard pressed to find anyone who can match his charisma and ruthless leadership style.

He was born into the nationalist struggle, learning the ways of armed conflict from his father Gerry, an IRA member who served eight years in prison for taking part in an ambush on a police patrol in Belfast. Home life wasn't easy and Mr. Adams later talked about his father's sexual and physical abuse of family members, saying his dad died "a very lonely man where he should have been surrounded by loving family members."

Mr. Adams quit school as a teenager, taking a job at the Duke of York pub in Belfast and lapping up the political conversation and simmering Catholic anger. He soon became a fixture in Catholic civil rights marches, joining the movement to end discrimination against Catholics by the Protestant majority. His protesting landed him in jail in the early 1970s when he was swept up without charge under special powers the police used to round up suspected IRA members. He was eventually acquitted of being an IRA member but questions have always persisted about his role in the organization.

In the early 1980s he turned his attention to politics, becoming leader of Sinn Fein and winning a seat in the British parliament, although he refused to set foot in Westminster as a protest over Britain's rule in Northern Ireland. He gained national prominence in the 1980s when prime minister Margaret Thatcher banned radio and television channels from broadcasting his voice. Suddenly Mr. Adams was the undisputed leader of republicanism and one of the country's most recognizable figures. The public attention also made him a target of Protestant paramilitaries who once sprayed his car with bullets hitting his arm, shoulder and neck but failing to kill him.

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As his popularity grew, Mr. Adams refashioned Sinn Fein and expanded it into Ireland. He eventually gave up his parliamentary seat in Belfast and won election to the Irish Dail, further proof of his determination to make Sinn Fein an all-island party. By 1994, he had the political clout to push the IRA into a ceasefire, setting the stage for the 1998 Good Friday Agreement. That eventually led Sinn Fein into a power-sharing government in Northern Ireland with the main Protestant party, the Democratic Unionists.

"History worked in Gerry's favour," said Mr. O'Doherty. "He knew the drift of things. He knew the way things were going." Mr. O'Doherty pointed out that Mr. Adams benefited from changing demographics. The Catholic population in Northern Ireland soared in the 1990s and 2000s, rising from about one-third of the total population to nearly one half. "In a sense, whoever was leader of the nationalist movement was going to get the feeling that history was moving in that direction," he said.

Mr. Adams has also shifted his political views to suit the changing times, dropping his hardline socialist rhetoric and embracing the capitalism and free market of the European Union. He's also become a supporter of greater access to abortion and same-sex marriage, putting a strain on the party's Catholic base. And for all his political success, Mr. Adams has run a tightly controlled organization, pulling virtually all the strings and leaving little room for dissent. Ms. McDonald faced no challengers for the leadership, a sign that Mr. Adams had anointed her as his successor.

For Ms. McDonald, the challenge now is to move out from Mr. Adams's shadow and pull the party into the future on both sides of the border. That won't be easy. Sinn Fein is deeply divided between its northern and southern branches and the IRA affiliation won't fade quickly. Last month Barry McElduff, a Sinn Fein member of Parliament, resigned after appearing to mock the anniversary of an IRA massacre in 1976 that killed 10 Protestants. But it took days for Mr. Adams and other Sinn Fein leaders to respond to the growing outcry over Mr. McElduff's actions, raising questions about their judgment. The party's leader in Northern Ireland, Michelle O'Neill, also has close ties to the IRA and regularly attends commemorations that infuriate unionists. As leader of both branches of the party, Ms. McDonald will have to tread a fine line between distancing herself from Ms. O'Neill's overt displays of IRA solidarity, which are popular among republicans, and setting Sinn Fein apart. And even more concerning is the political stalemate in Northern Ireland that has left the province without a functioning parliament for more than a year because of a dispute between Sinn Fein and the DUP, something that Ms. McDonald will now have to resolve.

"Mary Lou McDonald's job is not easy in that she is being asked to keep a party together that has quite different bases on either side of the border. In Ireland it is a radical, left-leaning, anti-establishment party, but more mainstream and conservative in Northern Ireland," said Eoin O'Malley, director of the School of Law and Government at Dublin City University. "She's also had to deal with a lot of accusations of bullying in the organization. It will be hard to present the party as modern, liberal and in favour of equality, when so many members complain that the organization is hierarchical and with a culture of violence."

Ms. McDonald has promised to cut her own path as leader and she'll be hoping that her track record and experience will translate into gains in the Irish republic, where Sinn Fein is running a close third behind the governing Fine Gael and opposition Fianna Fail. Last month she told the party faithful: "I won't fill Gerry's shoes. But the news is that I brought my own. So I will fill my shoes, I will walk in my shoes."

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She may not be walking alone. Mr. Adams has already hinted that he isn't fading away. "In their rush to write my political obituary some in the media have concluded that I'm now to retire," he wrote in a recent blog post. "Well, they're wrong. I will continue to serve the republican struggle and Sinn Fein, if and when I can."