Many years ago a German missionary priest, with little knowledge of the country’s history, came to Ireland seeking vocations. At a boys’ school in Drogheda he said how honoured he was to speak in “a town so closely associated with Blessed Oliver Cromwell”. And holding up a leaflet, he told the boys that it had “a prayer on the back for the canonisation of Blessed Oliver Cromwell”.

More recently, attempts have been made, while not to canonise, at least to humanise the man who has been called England’s Hitler. Two such works are by Catholic historians, Antonia Fraser in 1973 and Rev John Morrill last year. In the 350 years since his death nearly 170 biographies of him have appeared. Over 250 roads in Britain bear his name; there are Cromwell museums and a Cromwell Association. One historian, Samuel Gardiner, called him “the most typical Englishman of all time”. Surely not.

Another Catholic historian, Hilaire Belloc stated, “Official English history has lied more about Cromwell than anybody else. It has presented him as a bluff, middle-class person truly representative of the English people.” In fact he came from a very wealthy family and was connected with “about a dozen millionaires of his day, whose huge fortunes came entirely from the loot of the Catholic Church”.

His great-grandfather, Morgan Williams, a Welshman, married a sister of Henry VIII’s powerful minister Thomas Cromwell, who confiscated the monasteries. In gratitude, one of Williams’ sons changed his name to Cromwell and Oliver sometimes signed his name “Williams alias Cromwell”, as on the receipt for his wife’s dowry.

The only boy among eight children, he was born in 1599 in Huntingdon, 60 miles north of London. He first encountered Puritanism at the local school, whose head-master was bitterly anti-Catholic, and later in Cambridge University. He was there only a year when his father died and he had to return home at 18 to manage the estate and run the household. At 21 he married Elizabeth, daughter of Sir James Bourchier, a wealthy London businessman. They had nine children, five boys and four girls.

Little was heard of him for the next 22 years, except that at 29 he was MP for Huntingdon and that he became more involved in Puritanism, preaching regularly at house meetings. Inspired by Calvin’s extreme Protestantism, it first emerged in Britain as Scottish Presbyterianism and spread south to England as Puritanism. It rejected not merely the divine authority of the pope but also that of bishops and the priesthood, claiming that all baptised laymen were priests.

For Cromwell and the Puritans the Bible was the only true source of faith and morals and it was to be interpreted individually. He believed in an avenging God, the uselessness of good deeds, the damnation of most people and the predestination to glory of the few – he, of course, among the elect.

Cromwell was, said Freud, a suitable subject for psychoanalysis. From childhood he showed symptoms of a neurotic condition which later developed into what in modern psychiatric parlance is called “manic depressive psychosis”. He first went for mental treatment, when he was 29, to Sir Theodore de Mayerne, a London specialist. Soon after this he seriously considered emigrating with his family to a new Puritan colony in Massachusetts, North America. But, unfortunately, he changed his plans when his widowed and childless uncle Sir Thomas Steward died leaving him a large fortune.

It was at the start of the Civil War, of parliament against the royalist supporters of the Protestant King Charles I, that Cromwell first became prominent. He organised a group of 60 cavalry to join the parliamentarian army at the battle of Edgehill in 1642 and soon became army leader in East Anglia. A year later he became General and led the cavalry in its decisive victory at Marston Moor, 7 miles from York, in the War’s greatest battle, making him its most famous military figure. He ascribed his victory solely to God, thus starting the process by which he saw himself as specially blessed by God.

In 1645 he had all the military forces reorganized into what became the New Model Army. It first proved itself in June 1645 in Cromwell’s victory at the battle of Naseby, which ended the First Civil War. He was rewarded by parliament with great estates, yielding vast revenues, in England and Ireland. Now the most powerful man in Britain, he met King Charles a few times in 1647, but no agreement was reached.

During the Second Civil War, which began in April 1648, Cromwell led part of the army first in Wales and later in Scotland and soon ended the War. This time he determined to kill the King, despite the efforts of some MPs and others to spare him. He had Charles I beheaded on 30 January 1649, which caused widespread revulsion. Looking down on the dismembered royal corpse, he murmured, “Cruel necessity”.

A few months later Cromwell brought 20,000 soldiers to Ireland, landing at Dublin on 15 August. Describing his mission as a Protestant crusade against the barbarous Irish Catholics, he stated, “It is the priests with their Mass who are the intruders in Ireland. We come to maintain the glory of English liberty in a nation where we have an undoubted right to do it. Blood and ruin shall befall those who have cast off the authority of England and I shall rejoice to exercise the utmost severity against them.”

He began doing so in Drogheda, where the royalist garrison of 2,520 Catholics was led by Sir Arthur Aston, an English Catholic. “After a stiff resistance,” writes Antonia Fraser, “Cromwell in a white heat of passion ordered all to be killed, including Aston, who was bludgeoned to death with his own wooden leg.” Over 1,000 old men, women, children and all the priests also perished. Dr George Bate, Cromwell’s personal physician, stated in his memoirs that about 4,000 Catholics were killed in Drogheda.

Cromwell next gave a repeat performance in Wexford where, in his own words, “not many less than 2,000 of the enemy were killed”, including again 1,500 of the town’s old men, women, children and priests. This savagery frightened many of the 25 towns and castles in Munster to surrender when Cromwell marched on them, though some 2,000 of his soldiers were killed taking Clonmel and he failed to take Waterford. When New Ross’s governor asked for freedom of conscience for its people, Cromwell replied, “If you mean liberty to exercise the Mass, that will not be allowed.” W.C. Abbott, one of his biographers, said “there was something near paranoia in Cromwell’s attitude to Catholic clergy.”

"Cromwell lived on Irish soil for only nine months,” says the Oxford history professor Roy Foster, “but few men’s footprints have been so deeply imprinted upon Irish history.” When he returned to England, via Youghal, he was feted as a hero and “a day of public thanksgiving” was declared for his campaign in Ireland. He left his sons-in-law Ireton and Fleetwood behind to finish his Irish campaign, during which 400,000 Irish were deported, killed or died out of a total population of about 2 million.

According to the subsequent Cromwellian Land Settlement, 11 million of Ireland’s best 20 million acres were confiscated and given to 7,500 of his soldiers, instead of pay, and to 1,043 English adventurers at low prices. Many Irish Catholics were deported and none were allowed to own property except in Connacht, thus giving 80 percent of the land to Protestants.

Back in England Cromwell soon established a Commonwealth or Republic and, deciding against becoming king, ruled as Lord Protector or dictator, living in splendour in London’s royal palaces. He imprisoned opponents, without trials, in special camps in the Scilly Isles and the Isle of Wight. He abolished any Catholic influences that survived in the new Anglican Church, like bishops, celebrating Christmas and “scandalous pictures” of the Virgin Mary.

The few English Catholics who survived the Tudor persecutions – about 60,000 out of a total population of five million in 1640 – were treated much more harshly. Those who refused to take the explicitly anti-Catholic oath of Abjuration had two-thirds of their property confiscated and, having no churches, if caught attending Mass in private houses were fined “for popery”. Alison Plowden gives a graphic account of life under Cromwell’s dictatorship in her new book, In a Free Republic.

Cromwell was 5ft 10in in height, with “a swollen and reddish face, small beard, long hair, a large red nose and facial warts”. He famously told the artist Lely, “Paint me warts and all or I won’t pay you a farthing.” Living in constant fear of assassination, he always carried a loaded musket.

His health declined quickly and he died from malaria and pneumonia on 3 September 1658, aged 59. He was secretly buried in Westminster Abbey, but three years later, on the Restoration of the monarchy, his body was taken to Tyburn, where it was publicly hanged for some hours and then decapitated.

His head was displayed on a spike at Westminster Hall for about 20 years. His body is believed to have been buried in the privately-owned Newburgh Priory, Yorkshire; and a head, said to be his, was buried in a biscuit tin in 1960 at Sidney Sussex College, Cambridge.

Thomas Carlyle, the 19th British century historian, called Cromwell “a true giant of a man who brought Protestantism to its most heroic phase”. So maybe that German priest in Drogheda was right after all!