Conor Cruise O'Brien, the greatest living Irishman, is 90 today. He has been at the centre of so much controversy in his life that few would have predicted that he would get to the point where he will most likely die in his bed. After all, as he once proudly asserted: “I intend to administer a shock to the Irish psyche.” He has succeeded triumphantly in this. Above all, in the 1970s, he shattered the lazy intellectual consensus in the Republic in support of irredentist nationalism.

The “Cruiser”, as he is known to his friends, loves “mixing it” - whether as a UN envoy, Cabinet minister, biographer of Edmund Burke, historian of Israel, novelist or playwright. In Ireland, as in England, there is still an idea that a life of the mind and a life of action are not quite compatible. In that sense his career has been more continental, echoing 19th-century statesmen and men of letters.

O'Brien's life has spanned the entire existence of the Irish State: his first memory is of the sound of Michael Collins's pro-Anglo-Irish Treaty forces bombarding republican anti-Treaty elements in the Four Courts Building in Dublin in 1922, which signalled the start of the civil war. He was born into the inner sanctum of intellectual Home Rulers - the supporters of Parnell and Redmond who wanted devolution within the Empire rather than separation. Indeed, his mother's family, the Sheehys, appear in veiled form in Joyce's Portrait of the Artist as a Young Man.

This cosmopolitan world was dealt the sharpest of blows in the aftermath of the 1916 uprising. Post-civil war Ireland was a suffocating, monoculturally Catholic backwater, in which the O'Briens and the Sheehys were no longer the “coming men”. Their place had been taken by the newly ascendant political class of Sinn Fein-IRA.

O'Brien has been a standing rebuke to the mores of traditional Gaelic nationalism from the moment that his agnostic mother sent him to the predominantly Protestant Sandford Park rather than a Catholic school. But despite this disadvantage, he proved too good to ignore.

He joined the Civil Service and soon acquired two improbable patrons in the ruling Fianna Fail party - Éamon de Valera and his Foreign Minister, Frank Aiken, a former IRA chief of staff. The latter gave him his first big break by seconding him to the UN. There, Dag Hammarskjöld, the Secretary-General, sent him as his personal representative to the newly decolonised Congo, in the first of the UN's peace-keeping missions. Katangese rebels were leading a breakaway state backed by the outgoing Western imperial powers against the central authorities led by Patrice Lumumba.

O'Brien was a vigorous anti-imperialist and sought to uphold the territorial integrity of the new Congo. But such was the big power disquiet that O'Brien was recalled from Africa; the result was To Katanga and Back, a memoir of the intrigues, still considered a classic. Such was his reputation as a progressive that he was soon asked by the ruler of newly independent Ghana, Kwame Nkrumah, to become vice-chancellor of the national university (he also adopted two black children with his second wife).

Anti-imperialism proved critical to his emerging revisionism on Northern Ireland when the Troubles broke out in 1969. The “Cruiser” contended that although the Republic went through the rhetorical motions of deploring republican violence, there was also a deep ambivalence about clamping down on it effectively. That owed much to national myths about centuries of British and Protestant oppression. Or, as he observed after the Official IRA bombed the headquarters of the Parachute Regiment in 1972: “Were the seeds of Aldershot sown in an Irish classroom?”

He ceaselessly explored these themes, most notably in States of Ireland (1972), turning the conventional wisdom of the South upside down. It was not the British presence in Northern Ireland that was colonialist; rather, it was the South's aspiration to rule over a million Unionists. So who was the oppressor now?

Above all, O'Brien feared for what might become of the southern state if it were sucked into a tribal adventure to rescue northern nationalists; he discerned the prospect of renewed civil war. He may have been a liberal about the social nature of the Irish State, but when it came to its core stability he was anything but. He put this into practice as Minister of Posts and Telecommunications in the Fine Gael-Labour coalition of 1973-77.

O'Brien became a hate figure among republicans and civil libertarians for his modernisation of Section 31 of the old Broadcasting Act giving the Government the right to ban paramilitaries from the airwaves; it was precisely at times of heightened tribal consciousness, he argued, that restrictions on speech were necessary.

What remains? O'Brien is still to complete his study of George Washington, expecting, by his own testimony, to die with his pen in his hand. What a lethal instrument it has been. In words attributed to George Orwell: “We sleep safe in our beds because rough men stand ready to visit violence on those who would do us harm.” The “Cruiser” remains the roughest of intellectuals - and these islands are the better for it.

Dean Godson is Research Director of Policy Exchange

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