I think of him as one of the great writers, great historians, great personalities of the twentieth century, no less. Not everyone agrees: here's a just slightly grudging obit from the Irish Times. Arts and Letters Daily has others.
In the early 1970s, Conor Cruise O'Brien did something profoundly wise, courageous, and liberating. He became virtually the only non-Protestant Irish person of any prominence, and one of the few left-leaning and progressive people in the world, to declare that the IRA and all its fronts and allies were disgusting, criminal, stupid, a fraud upon Irish history and politics, and self-defeating to the point of insanity. Of course he was right, but he got very little thanks for saying so, then or later.
I read his States of Ireland soon after it was published in 1972, and it has stayed with me ever since, one of the foundations of my political thinking even. It's partly journalism, and may not have held up entirely as the headlines of the day faded, but it's still a wonderful book not just about Ireland, but about not going along with fashionable murderers any and everywhere.
My 1867, too, is deeply endebted to his Edmund Burke biography The Great Melody, a book I would keep on even the shortest shelf of best history books. Another damn good book is Donald Akenson's biography and anthology of a few years ago, Conor.
I'm happy to say I met him once, at a dinner at Massey College, and once received a letter from him (about 1867, which a mutual friend had sent him).