First I have to declare an interest. James Downey's new biography of Brian Lenihan is published by New Island Books, who are also the publishers of my own recent book. So, naturally, my first wish for Jim is that his book will shoot straight to No 2 on the bestsellers list!

Seriously, James Downey has a well-established reputation as a literate and literary journalist, with a keen eye, an encyclopaedic knowledge of politics, and a high level of insight into its inner workings. None of those qualities lets him down here. This is a first-rate account of a life lived well, and of duty done to the highest personal standards.

The book is particularly good in its description of the forces that shaped Brian Lenihan, and the Ireland he grew up in, in the earlier years of the State. It's easy to see how a family that grew up in those circumstances was imbued with a passion for politics that has led to five of the Lenihans becoming members of Dail Eireann, surely a record, and one of them (Brian himself) having the unique distinction of serving under every leader of Fianna Fail, including a total of 24 years in ministerial office.

I couldn't claim to know Brian Lenihan well, certainly not to the extent that I was ever allowed to see behind the mask he wore and detect the erudition and intellect behind the clown's face. Those who knew him better knew that there were two men, the bluff, sometimes bumbling character who always allowed himself to be wheeled out to defend the indefensible, and the well-read, articulate politician who became an instant master of every brief he was given. In his chosen political home, he may well have felt that critical analysis would never be preferred to "Up Dev".

I did have an opportunity once to see Brian Lenihan's grace under fire. On the night Mary Robinson was elected I was able to watch Brian Lenihan, after what must have been the most traumatic political week of his life, accept defeat with the kind of grace and dignity that is all too rare. I can still remember thinking how much he deserved to be admired at that moment.

But there were other moments, and other times. And here I have to enter the first caveat. One of the things I've never noted before in Jim Downey's analysis is a high level of forgiveness, and, broadly speaking, he's right to be sparing in his forgiveness. He would be the first to say, and (most of the time) I would agree with him, that politics has to be forced to operate to the highest standards, and doesn't deserve forgiveness when it fails to measure up.

But Jim forgives Brian everything. The picture that emerges is of a man who could do no wrong, but was surrounded by others just as venal and small as Jim would expect them to be.

The portrait of Jack Lynch that emerges, for instance, is very harsh, and not nearly rounded enough. There is a sustained sense of outrage at the very existence of the Progressive Democrats, and they come in for particular abuse in connection with the events that surrounded Lenihan's sacking from the Cabinet in 1990.

This is unfair, and prejudiced. Des O'Malley and the PDs didn't create the problem caused by the Jim Duffy tapes. They were in an impossible situation, and they were entitled to look to Fianna Fail for a solution. It was Charles Haughey to whom it never seemed to have occurred that he, rather than his Tanaiste, ought to have been the sacrificial lamb. For that reason, I tend to agree with Jim Downey when he says that, of everything CJH did, the worst of all was his betrayal of Brian Lenihan.

But, and here I also believe Jim Downey has it wrong, Brian Lenihan contributed to his own downfall in that controversy. In a rather convoluted passage, Downey seems to me to exonerate Lenihan from the charge that he had lied over the famous 1982 phone calls to President Hillery. But it's not possible to exonerate someone who gives two diametrically opposite accounts of the same incident. Inevitably - something Jim forgets to mention - one of them has to be untrue.

The same conundrum applies to a lot of Brian Lenihan's life, and it's not possible to explain it away by saying that the party always came first. He knew he was wrong in seeking to undermine the Anglo-Irish Agreement on Charlie's instructions, but he went ahead and did it anyway. He defended too many indefensibles, when he knew in his heart that he should have been trying to change the decisions he was defending.

AND, most of all, this was a man who by all accounts was as financially honest as the day is long. In fact, he had no interest in money and neglected opportunities to make himself honest money, let alone the other kind. How is it possible that a man like this, his own integrity unchallenged and undoubted, never thought to ask the question that troubled so many others about his friend, the friend who sacked him and destroyed his career, Mr Haughey?

More than any other figure in the Fianna Fail of his time, he would have been listened to if he had stood up and said enough is enough. But he never did, and he was a lesser man because of it.

He believed, apparently, that party was all. Big parties, in his view, were the bedrock of democracy. Strong government demanded strong parties.

It's a viewpoint shared by many in Fianna Fail, along with old Stalinists, Catholic bishops and Argentinian generals. This ideology, apparently approved of by Jim Downey, is not just a cover-all for "my party right or wrong". It's dangerous, and it may have been the only bit of Margaret Thatcher's philosophy that Brian Lenihan shared, however thoughtlessly.

In the end, and this is where Jim Downey's personal fondness for Lenihan gets in the way of an objective appraisal, Lenihan was a man who sold himself short. He had the talent to be much more critically important than he was, and he deliberately chose never to display that talent. He was honest, and decent, and often thoughtful (politically and personally). But he kept the best of himself hidden from view, even when his country needed to see his best side.

Perhaps the most damning criticism that can be made of any politician is that he could have been great, and chose to be popular instead.

Lenihan, His Life and Loyalties by Jim Downey is published by New Island Books, price £9.99; 251pp including index and select bibliography