The arresting sentence in the last chapter of Malachi O’Doherty’s Gerry Adams – An Unauthorised Life reads, “Gerry Adams is one of the most successful politicians in the democratic world”.

It is striking because O’Doherty and Adams have form with each other. They are contemporaries: O’Doherty is 66, Adams, 68; both are from republican backgrounds; both were raised in west Belfast; and, it seems fair to say, both have a robust disregard for each other.

Sinn Féin in Northern Ireland would know O’Doherty well as a commentator and as a thorn in its side, a journalist fiercely critical of the IRA’s “war” and of how Sinn Féin pursues its political ambitions. Once, at a press conference outside Castle Buildings, Stormont, O’Doherty said to Martin McGuinness: “If you shot me would that be a breach of the ceasefire, or wouldn’t it?”

That was after the July 1999 murder of Charles Bennett in west Belfast, allegedly at the hands of the IRA at a time when the IRA said it was on ceasefire, a killing that was morally questionably finessed to preserve the powersharing Belfast Agreement of the previous year.

McGuinness’s response, if memory serves, was, “Malachi, would you give my head peace”.

When O’Doherty put in a request for an interview with the Sinn Féin president for this biography he says he got a swift reply from the party press office: “Mr Adams wants nothing whatever to do with this project.”

So it comes as a surprise that O’Doherty managed to objectively view the Sinn Féin president as one of the world’s most successful politicians.

“Well, he is. How do you measure success? You measure it by how you enlarged the vote for your party, and you measure it by your survival in office. And by those measures he is outstanding, he is amazing, there is nothing comparable.”

There’s a pause, and then he adds, “It does not mean he is a good person.”

“I don’t hate him as much as some journalists do but I think he is vain to an extraordinary degree,” O’Doherty continues. “I think he has an ego the size of the moon. He will not baulk at very cynical strategising. I presume it is true that he was a member of the army council of the IRA and that therefore he assented to enormous amounts of violence.”

He prefers not to name the journalists he feels hate Adams although a name or two come to mind. O’Doherty says he is not engaged in “some obsessive mission” against Adams. But as he talks he reflects and qualifies: “There is something in this culture that is slightly wary of being seen to be obsessive, even though what the Provos did was completely unwarranted and despicable. There is something in this culture that makes us hesitate about being wholly condemnatory or using words like ‘evil’.”

These are O’Doherty’s more recognisable and more personal feelings about Adams and the IRA but in the book he generally keeps them in check. The narrative travels from Adams as a toddler up to the present. In between we get the story of his life and how it shaped him – in so far as any writer can define Adams or pierce through to his core.

In this interview O’Doherty tells of seeing a photograph, that he wasn’t allowed use, of Adams as a boy aged about 12 with a group of five other boys. “They were all wee scruffs except wee Gerry with the neat clean crisp shirt and the hair neatly combed. He just looked so much like a Mummy’s boy sent out to do his family proud.”

The book further recounts how Adams’s life might have taken a different route. When he was 15 he told his father he believed he had a vocation for the Christian Brothers. According to O’Doherty’s research, his father’s response was that it would suit him better to join the IRA and fight for his country.

Of the Adams-McGuinness leadership combination he views the late McGuinness as the “junior partner”. In relation to the current crisis which came to a head when McGuinness resigned as Deputy First Minister in January, O’Doherty offers: “People see McGuinness as the guy who tried to make reconciliation work and see Adams coming on the scene as the guy who spoiled it all.”

The book also explores how Adams never deviated from the republican tradition of his family. It addresses his rise, allegedly, through the IRA and deals with the IRA Troubles campaign, of so many killings, Jean McConville, the Disappeared, the murder of Mary Travers as she left Mass in south Belfast, the Provisional IRA and Official IRA feuding, and the loss of his friends.

O’Doherty can see no evidence that such experiences and incidents caused trauma in his subject’s life, as they would many people. “He is insulated internally in some way against quite grotesque horror. He can keep on going,” he says.

“I think he has personal qualities you and I probably would not even be able to comprehend. I don’t think most people could sustain the amount of opprobrium that he has faced, and I don’t think most of us could sustain going to the number of funerals he has been to, the number of people close to him who have died and been killed.”

It took O’Doherty three years to complete the book. That was partly due to a first draft which his Faber and Faber editor told him veered between a straight “J’Accuse and a more dispassionate look” at Adams. “Which is it? You have to make up your mind,” his editor told him. “So I went for a more detached, a more measured examination of Adams. It meant, if you like, culling out some of the cynicism.”

O’Doherty, as you would expect, deals with Adams’s denials that he was ever in the IRA, citing numerous pieces of evidence that point to the contrary. He refers to Adams and McGuinness being brought on an IRA delegation to Downing Street in 1972; how he was “number one” on the British army and RUC special branch IRA list; how he negotiated a truce between the Provisional IRA and Official IRA in the 1970s; and much more besides.

But then he posits another interesting perspective based on interviews with former IRA members, some of whom would not be too sympathetic to the Sinn Féin leader.

One told him how when Adams was in prison in Long Kesh he “turned up for a lecture on how to strip an Armalite rifle and the other republican prisoners all laughed at him”.

“There is a sense that dyed-in-the-wool IRA men don’t believe he was an IRA man. [Former IRA man] Tommy Gorman is very frank in saying he was not really one of us. He wanted to bring guns into Ballymurphy [in west Belfast] and confront the army in 1970 but Gerry was against this,” says O’Doherty.

O’Doherty adds that former IRA men told him that when the IRA smuggled in huge amounts of arms from president Muammar Gadafy’s Libya in the 1980s the hardliners were gung-ho to mount “a Tet-style offensive in Belfast” similar to the attacks launched by the Vietcong in south Vietnam in 1968. But, he was told, this was overruled by Adams and the IRA army council.

‘Armed propaganda’ O’Doherty argues the IRA did not fight a “guerrilla war” but rather was engaged in an “armed propaganda” campaign that was sold as a “war” until “Sinn Féin was big enough to take over”. And Adams was central to that strategy, he believes.

“The insight is that Gerry is essentially political at heart,” he contends. “We have been arguing for years whether Gerry Adams was in the IRA or whether he would ever own up to it. But I have met IRA men who said he was but that he wasn’t really one of us, he wasn’t of our type, he wasn’t a militarist.

“He was more political than military. In a kind of way he is telling the truth when he says he was not in the IRA because even the IRA say he was not really one of ours.”

Many Sinn Féiners will take issue with such analysis as they will with his contention that the completion of the republican process isn’t a “united Ireland but it’s Sinn Féin inheriting the legacy of constitutional nationalism”. Regardless, O’Doherty says Sinn Féin taking over as the dominant nationalist force in Northern Ireland with big ambitions also in the South is “by any measure an amazing achievement”.

Historian Prof Marianne Elliott in a blurb on the back of the book says that O’Doherty in the “impressively measured and stylishly written” biography “has gone further than most to disentangle” who is Adams.

Implicit in this compliment is that there is more to unravel and O’Doherty cheerfully concedes she is correct. “He is a very enigmatic character,” he agrees.

An Unauthorised Life discloses more than most other explorations of the Sinn Féin president; we get a deeper sense of the man. Of course, adds O’Doherty, the deepest revelations would come if Adams himself wrote a brutally honest account of his life.

O’Doherty says that if Adams hadn’t given his life to republicanism, he would have made a good writer or journalist. But he sees books and personal reflections that Adams has written as too sugar-coated and partly “propagandising for his own humanity”.

He doesn’t think Adams, when he finally decides to retire, would have the fierce honesty to write a tell-all story about himself. “I don’t think he could do it. He has the capacity to write a very charming and engaging memoir, to deploy devices of self-deprecation, but there would be too much sentimentality.”

Essentially, O’Doherty considers Adams a brilliant leader and cynical strategist and at heart a purist and conservative old-style republican in the tradition of his father and that previous generation of republicans. And he adds, “I don’t think he will ever let go of that ideological thrust. I think ultimately he is a true believer in the same way that Ian Paisley was a true believer. The faith is in the blood.”