Frank Duff was born in 1889 into a well-off Dublin family with roots in Co Meath, who was marked out for even higher social status by attending the elite Blackrock College.
An avid reader of high intellect, he passed the civil service examination and became an influential mandarin in the governance of the Home Rule administration, and afterwards in the Irish State founded in 1921. He served as private secretary to Michael Collins.
Well known and well liked by the State's two Founding Fathers, WT Cosgrave and Eamon de Valera, Duff did invaluable work in the departments of lands and finance. He looked destined to become Ireland's most senior bureaucrat.
However, on top of his day job, Duff, as a young man, joined the Vincent de Paul Society and was increasingly drawn to become a full-time religious worker. This calling consolidated with his reading of a book by a French saint, Gringion de Montfort, author of the Treatise on the True Devotion to the Blessed Virgin.
His realisation that "the book was true" propelled him to leave the civil service and establish in 1921 a new Catholic lay organisation known as the Legion of Mary.
Using his skills as a draftsman, Duff compiled a handbook that defined the legion as a voluntary body "at the disposal of the bishop of the diocese and the parish priest for any and every form of social service and Catholic Action which these authorities may deem suitable to legionaries and useful to the welfare of the church".
Until his death in 1980, aged 91, Duff, a life-long bachelor committed to celibacy, presided over a worldwide spiritual empire. Today, the Legion of Mary has an estimated four million active members -- and 10 million auxiliary members -- in close to 200 countries in almost every diocese in the Catholic Church.
On the ground rule of serving one's neighbour, members attend a weekly meeting and undertake each week to do "voluntary work of substance" in their community. Auxiliaries say certain prescribed prayers daily.
So phenomenal was the spread of the legion that Duff, the friend of popes, was honoured by being selected as a "lay auditor" in the 1960s at the final session of the Second Vatican Council in Rome.
There, he spent 97 days living with the Dominican Fathers of San Clemente. In his spare time he addressed numerous meetings of bishops and lay groups from around the globe. He was acclaimed as "the pioneer of the laity".
Now, 31 years after Duff's death, biographer Finola Kennedy has not only rescued him from obscurity for a new generation, she has thrown down the gauntlet to Irish historians, accusing them of ignoring his claims to greatness as "the founder of the largest international association that has originated in Ireland, certainly in modern times".
Dr Kennedy, a respected economist who was Duff's god-child, has obtained access to his vast correspondence kept in the files of the Legion of Mary's headquarters in Dublin.
This first full-scale biography of Duff is to be commended, in spite of Dr Kennedy's tendency for gushing devotion toward her spiritual hero.
She has resurrected his major contribution not just to the history of the modern Irish Church, but also to our appreciation of politics and society in 20th Century Ireland.
Dr Kennedy's weighty book provides abundant new material showing Duff's constructive approach also to the material development of the country.
Although Duff was renowned for his "docile loyalty" to church authority, the most revealing part of Dr Kennedy's biography is how he achieved world acclaim for the legion, in the face of decades-long obstruction from two successive archbishops of Dublin, Edward Byrne and John Charles McQuaid.
Known to his friends as "a practical St Francis of Assisi, combining poetry with good management", Duff, in his 33,000 letters, shows himself to be often fearless in speaking frankly to the imperious McQuaid. Yet, he never defied the iron-willed Archbishop.
For years Duff was withheld permission by McQuaid to publish an update of the legion handbook, even though in Rome he was being feted by Pope Pius XI and his Secretary of State, Cardinal Eugenio Pacelli, the future Pius XII.
It was only in the 1960s, when Pope Paul VI praised him at the Vatican Council, that for the first time McQuaid hailed him as Frank, and not as Mr Duff. This was in stark contrast to McQuaid's scheming in the 1940s to ruthlessly scuttle Duff's Mercier Society, which provided a forum for dialogue between Catholics and Protestants, and the Pillar of the Spirit, a platform for Catholics and Jews.
These gatherings had grated with McQuaid's obsessional hatred of "the so-called Reformation" and his anti-semitism, then a feature of the Holy Ghost Congregation, to which McQuaid belonged.
In launching Dr Kennedy's book, Archbishop Diarmuid Martin appeared to distance himself from McQuaid, the archbishop of his student days at Clonliffe College, when he lamented "the small-mindedness of part of the establishment of the Archdiocese of Dublin".
Dr Kennedy's research highlights how the Dublin clergy, indifferent to spirituality, feared an organisation which dared to promote mixed membership of men and women.
In profiling Duff as a man who loved walking and cycling -- and the company of women as well as men -- Dr Kennedy has exorcised his image as a religious fanatic, and shows that his case for canonisation is likely to be well received in Rome.
John Cooney, the Religion Correspondent of the Irish Independent, is the biographer of John Charles McQuaid, Ruler of Catholic Ireland.