Peter Hart considers that, "in the time allotted to him," Michael Collins "became the most ruthless, the most powerful, the most calculating and the most successful politician in modern Irish history." There is no disputing Collins's ruthlessness, power and calculation. Success is harder to measure. Collins's great onetime friend and rival, Eamon de Valera, in the much longer time allotted to him, was the most forceful presence in Ireland, through victory and defeat, from the Easter Rising of 1916 to the end of the civil war in April 1923; again from his coming to power in 1932 to his loss of power in 1948; and yet again when he returned as Taoiseach (prime minister) from 1951 to 1954 and from 1957 to 1959. It was de Valera, not Collins, who gave Irish people a sense of themselves as a nation, mindful of their history and mythology, their nearly lost language, their identity and difference. Even when he moved to the mainly nominal office of president of the Irish Republic, from 1959 to 1973, de Valera retained much of his symbolic aura and maintained his dream of "the republic." He was for good and ill a visionary, something of a scholar, a mathematician, nearly a poet; Collins was a pragmatist, determined to take one strong step at a time, without misgiving.

Michael Collins was born on Oct. 16, 1890, the youngest of eight children, to Michael and Marianne Collins, a farming couple in Woodfield, County Cork. At the age of 15, after a nondescript education, he managed to pass the post office's boy-clerkship examination and took up a job in London. He consorted only with Irish emigrants like himself, joined the Gaelic Athletic Association, played Gaelic football and hurling with no great distinction. In 1908 he joined Sinn Féin and the following year was sworn in as a member of the Irish Republican Brotherhood. Moving from one minor job to another, he apparently stayed in London till January 1916, when he went back to Ireland and - in April - took part in the Easter Rising. But he was not one of the leaders. On the collapse of the rising, he was arrested and interned in Frongoch, Wales, where for the first time he asserted himself among his peers.

Released in December 1916, he returned to Dublin and a few months later took a job as secretary to the Irish National Aid and Volunteer Dependents Fund, an association to help those who suffered for their participation in the rising. This was an ideal job for Collins at that moment, because it kept him in touch with men and women of similar convictions: he used these connections to set up a remarkably effective underground intelligence network. During the war of independence, from 1919 to 1921, Collins's spies infiltrated Dublin Castle, center of the British administration in Ireland. His "Squad," as it was called, was responsible for killing several senior detectives and members of the Dublin Metropolitan Police.

Not a natural-born killer, Collins killed for what he regarded as cause. On Nov. 21, 1920, the Squad executed a dozen British secret service agents. Fearless for his own life, though with a price on his head, Collins stormed through the streets of Dublin and thought himself invincible.

The aim of his insurgency was to make Ireland ungovernable and to force the British to call a truce, to begin with, and then to talk terms of peace, departure and independence. In September 1921, de Valera, as president of the Dáil (the provisional government), chose Collins, Arthur Griffith and five other men as the delegates to a peace conference in London. A question is often asked: Why did de Valera refuse to take part in the talks? Collins did not want to go to London without de Valera, but in the end he did. De Valera probably thought the talks with the British prime minister, David Lloyd George, would fail to achieve what he - de Valera - wanted: that the whole of Ireland would be granted its independence, and that the only tie with Britain would be "external association," whatever that meant.

What Collins, Griffith and their colleagues brought back from London was much less than de Valera's dream. They got dominion status for 26 of the 32 counties, with complete independence in domestic affairs and fiscal matters. They accepted under duress an obligatory oath of allegiance to the crown. Naïvely, they took seriously the promise of a boundary commission that might or might not propose to reduce the size of Northern Ireland - already established by the Government of Ireland Act of 1920 as the Six Counties - in consideration of the demographic, religious and cultural conditions in perhaps two counties, Tyrone and Fermanagh. The Irish delegates thought these Articles of Agreement were the best they could get, and they signed them on Dec. 6, 1921. They were accepted by a narrow margin of the Dail on Jan. 7, 1922. De Valera and the republicans rejected the terms. A civil war was inevitable. In April members of the Irish Republican Army occupied the Four Courts in Dublin. The government - the Irish Free State - did not respond till June 28, when it ordered its troops to shell the Courts and drive the I.R.A. out. In July, Collins became commander in chief of the Free State Army. One month later - flamboyant to the end - he was traveling in the back seat of an open car with a small convoy to protect him, when they were ambushed by a few I.R.A. men in a small valley in County Cork.

Collins was shot to death, probably killed by a ricochet.

Peter Hart's "Mick," a fine biography, concentrates on Collins's work, the tasks he took on for the associations he joined and ultimately for the provisional government: minister of finance, a job he carried out brilliantly; director of intelligence, the main source of his reputation as a hero, daring beyond description; and commander in chief of the army, in which he acted as if he had indeed an army to inspect in full order and battle dress.

According to Hart (the author of "The IRA and Its Enemies"), the secret of Collins's success is that he worked harder, and at more tasks, than anyone else. If de Valera was one of a kind, Collins was the perfection of a common kind. He did not have "the moral stature of a Daniel O'Connell, a Martin Luther King or a Nelson Mandela," but he had other qualities, short of greatness. Hart's book is written with immense verve, as if he wanted to acknowledge by the rapidity of his own style the relentless pace of the man he describes. He doesn't bother much with Collins's private life, his fiancée (Kitty Kiernan), his dalliances with women of high estate (Lady Hazel Lavery, Moya Llewelyn Davies, Lady Edith Londonderry), his drinking and carousing in London. There is plenty of such lore in Tim Pat Coogan's "Michael Collins: A Biography" (1990), one of the reputed sources for Neil Jordan's film "Michael Collins" (1996). Hart's account of the peace talks is especially good, but I wish he had explained more thoroughly why Griffith and Collins and the rest allowed themselves to be intimidated by Lloyd George and accepted his deadline, Dec. 6, 1921, for the end of the talks. The threat of sending in thousands of soldiers and destroying the Irish insurgents seems to me to have been a bluff.