IN THE DARK early-morning hours of November 19th, 1821, at a place called Gurtnapisha, near Mullinahone in Co Tipperary, a group of about 17 men surrounded the thatched house of a well-to-do farmer, Edmund Shea. They demanded that he hand over arms they believed he had in the house. When Shea refused or resisted, the attackers set fire to the house. Inside were seven members of the Shea family - five of them children - six male labourers and three female servants. All 16 were burned alive.

Almost two years later, near Kildorrery in Co Cork, a gang variously estimated at up to 60 strong attacked the house of another farmer, Thomas Franks. The Frankses were down-at-heel minor gentry who acted as middlemen for the dowager countess of Kingston and were regarded by their subtenants as being particularly cruel and avaricious. Thomas Franks had also given evidence against a member of a previous raiding party, who was convicted and transported. In September 1823 a brutal revenge was taken. The raiders shot Franks and his son Henry, then beat them and Franks’s wife, Margaret, to death with crowbars. The skulls of both men were “literally smashed to pieces”. The raiders then gathered around the bodies and stabbed each repeatedly with a pitchfork, “each handing it to the other to inflict a blow” and the leader urging each one to “Do your duty”.

Between these two incidents, also near Kildorrery, in February 1822, a group of women and children travelling in carriages or wagons was stopped on the road. They were the families of members of the First Rifle Brigade, which had been sent to quell disturbances in the area. Perhaps nine of the dozen women were gang-raped. The attackers seem to have been especially keen to establish that the women were Protestants. One of the assailants pointedly told his victim to let the riflemen know that “it was Captain Rock’s men” that did the deed.

These were just three of the most notorious episodes in the wave of extreme violence that swept through Munster and south Leinster between 1821 and 1824. It had its origins in, and took its name from, a local agrarian disturbance that broke out on the 13,500-hectare estate of Viscount Courtenay in Newcastle West in Co Limerick. Courtenay, who was flamboyantly gay, lived abroad and ran up huge debts. His popular local agent was replaced by an English lawyer, Alexander Hoskins, who set about raising rents and collecting arrears. Hoskins’s 19-year-old son was attacked by seven men on the road, shot and beaten. His killers then “danced and played upon the fife for about an hour”.

In the disturbances that led up to this event, a blacksmith called Patrick Dillane distinguished himself in the art of throwing rocks at Hoskins’s hired men and was, as he later testified, “christened Captain Rock by a schoolmaster ... by pouring a glass of wine on his head”. In the campaign of terror against agents, tithe proctors, “land grabbers” and their perceived allies that gradually spread through the southwest, Captain Rock would be the name attached to the lurid threatening letters posted on so many doors. Before the fury finally abated, more than 1,000 people had been murdered, mutilated or badly beaten.

Unless you have a particular interest in 19th-century Irish history you are unlikely to know any of this. In the litany of Catholic and/or nationalist revolts, the big years are all familiar: 1641, 1690, 1798, 1803, 1848, 1867, 1916. Yet, after James Donnelly’s monumental and deeply revelatory study, the Rockite rebellion of the early 1820s has to be added to the list.

That it was indeed an organised rebellion, rather than simply a series of localised agrarian outages, is the case that Donnelly, retired professor of history at Wisconsin-Madison, makes in this superb book. Anyone wishing to argue otherwise will have to deal with one of the most formidable pieces of Irish historical research in recent decades.

It is easy enough to understand why the Rockite upheaval has been left out of the official narrative of Irish revolts. It did not have a middle-class urban-intellectual leadership. It has none of the romance or nobility of Robert Emmet’s speech from the dock or James Connolly’s execution. Its violence is utterly shocking: women and children were murdered, rape was used as a weapon of terror and revenge, and bodies were often mutilated. (In at least three cases the victims were decapitated.)

Above all the Rockite rebellion is hidden because it was nakedly sectarian. The revolt was undoubtedly shaped by rational economic factors: the slump in agricultural prices that followed the end of the Napoleonic Wars and a series of bad harvests that led to famine and a typhus epidemic in the years before the violence erupted. These tribulations especially affected the young, who made up the bulk of Captain Rock’s foot soldiers: Donnelly reckons that 70 per cent were under 30.

But Donnelly argues convincingly that the extremity of the violence was also fuelled by a millenarian anti-Protestant fervour. Rockism was a religious cult as well as an economic revolt. Though respectable Catholic opinion, led by Daniel O’Connell, did its best to cover up this dimension of the violence, it is clear that the ideology of the rebels was defined by the exotic watchword of Pastorini.

Signor Pastorini was the pen name of Bishop Charles Walmesley, whose reading of the Book of Revelations led him to predict in the 1770s that God’s wrath would be poured out to punish Protestant heretics in 50 years’ time. During the famine and fever of 1817, condensed versions of Pastorini’s prophecies began to circulate very widely among rural Irish Catholics. The idea that Protestants would be wiped out by 1825 gained a powerful hold.

The religious dimension of the violence was not, of course, entirely irrational. Much Rockite action was aimed against the oppressive levying of tithes by the established church on poor farmers who owed it no allegiance. There was also a rapid growth in aggressive Protestant proselytising in these years, to which the anti-Protestant rhetoric of the Rockites could be seen as a reaction.

This reaction, however, went far beyond the rational. The millenarian fervour helped to dehumanise the victims of the violence and to sanction atrocities against people who were, in any case, already doomed to extinction by the wrath of God. The threatening notices of which Donnelly makes brilliant anthropological use contain phrases like “I am only an instrument in the hands of the Almighty” or the exuberantly apocalyptic “Vesuvius or Etna never sent forth such crackling flames as some parts of Donoughmore will shortly emit”. (To be fair, these notices are not without a certain black humour. One intended victim was warned that “in compassion to your human weaknesses and in consideration of the enormous weight of your corpulent fraim [sic], I mean to rid you of these inconveniences by a decapitation”.)

With a binding ideology, its own system of oaths and laws and its successful use of agents to spread agitation from one county to another, the Rockite movement was a formidable force. Though never free of class divisions, it did manage at times to command allegiance not just among the poor tenant farmers, labourers and artisans but also among some middling and strong farmers. At is height in north Cork it could bring thousands of men and women into open insurrectionary action against troops and yeomanry. In January 1822 there were even daytime attacks on the towns of Millstreet and Newmarket.

The outbreak of famine in 1822 stalled the movement, but it took another two years for the authorities to fully repress it. The Rockites had no military victories, but their campaign of arson and murder did succeed in cowing the local magistrates, stopping the collection of tithes and lowering rents. The intimidation of witnesses and “informers” created a period of virtual legal immunity.

It took large-scale military occupations, mass hangings (about 100 prisoners were executed), transportations, the introduction of the brutal Insurrection Act, the suspension of civil liberties (most of those tried and sentenced were accused of nothing more than breaking the curfew), assisted emigration from Cork to Canada and an upturn in the economy to end Captain Rock’s three-year reign of terror.

In probing this remarkable episode so thoroughly and acutely, Donnelly has also given us a fascinating anatomy of pre-Famine peasant society. As an exposure of a hidden mental universe, an exploration of the roots of a particularly psychotic strand in Irish Catholic nationalism and a reflection on violence itself, Captain Rock is as important as it is startling.