A long-standing thorn in their side, his assassination, claimed by the IRA (their final killing before the August 1994 ceasefire), prompted undisguised delight over garda radio airwaves. "Tango One is down," was one frantic message; "Who murdered TC [Top Cat]?" called another; "Who cares?" came the jubilant reply.
Boorman, an Irish resident, directed the film, and his screenplay draws on Dublin crime reporter Paul Williams's unromantic biography of Cahill, which earned crime literature's ultimate accolade - the book most frequently stolen from Dublin's bookshops.
Williams documented how Cahill observed an unlikely criminal lifestyle, spurning gambling, cigarettes or drink. His one indulgence was expensive motorbikes and a peculiar domestic arrangement by which he remained married to his wife while also having a public relationship with her sister. His favourite pastime was racing pigeons.
Cahill's curious life has led to two other films in preparation: Ordinary Decent Criminal from Irish production company Little Bird, directed by Gerry Stembridge; and the BBC's Cast A Cold Eye, due to be screened in late summer, featuring some elements of his story.
Cahill the robber differed from traditional practitioners of straightforward jump-overs (literally, jumping counters to raid tills). An early grievance over an alleged garda frame-up led him to direct his efforts as much at embarrassing the enemy in blue as amassing wealth.
This theme is a core element in Boorman's film. In real life, he kept files on his main garda persecutors. He developed a cunning alibi technique, turning up at a garda station claiming he had been told to present his driving licence. While there, his gang would be carrying out another raid.
Irish Times reporter Padraig Yeates, who knew Cahill from the early Eighties, wrote: "He pursued his vendetta with the garda to incredible lengths. Like many criminals he could feel genuine moral outrage at the notion that gardai might plant incriminating evidence or give false testimony in a courtroom, without ever thinking such indignation was inconsistent with his own modus operandi."
Determined to retaliate in novel ways, Cahill allegedly dug up the greens at the Dublin garda golf course and slashed tyres of vehicles in the car park. Tyre-slashing became his signature, sending an intimidating message to would-be informers among neighbours. On the first anniversary of his death, south Dublin residents got a repeat treatment as a reminder.
Another twisted form of entertainment was to slip into neighbours' empty houses and then, sitting on his roof, abuse them about embarrassing things he claimed were in their safes. His mordant wit, well transferred to the screen by Boorman, had him turn up for a court appearance in a Mickey Mouse T-shirt and shorts with another pair over his head.
Sending up the law led to his creating "Concerned Criminals Against Drugs" in the early Eighties (imitating "Concerned Parents" groups then springing up across heroin-ravaged inner-city estates). CCAD staged a city-centre demonstration with Cahill among several participants wearing masks. An underlying concern was that emerging drugs gangs were threatening to draw down unwanted detective activity on the humdrum business of "ordinary decent criminals" trying to earn a living from armed robbery.
Criminals admired Cahill the master-plotter. His career stretched back to 1958, developed through reform school and jail terms for receiving, and climaxed when he masterminded the 1986 theft of priceless Old Masters (so well known they proved unsaleable) from Russborough House, the home of the late Sir Alfred Beit, the De Beers heir, in County Wicklow.
The next year, he broke into the offices of the Irish Director of Public Prosecutions and stole valuable files, presumably tracing witness names. That menacing trait had been seen in 1982, when Cahill was blamed for a bomb put in the car of Dr Jim Donovan, the state's forensic science laboratory chief and a witness in pending trials.
The film may fan garda anger on two fronts. One is the overall implication that their efforts to have Cahill convicted of serious offences were heavy- handed and less than competent. Some incidents in the film have the defence of being true. These include the Keystone-vintage Wicklow Mountains car chase, when Cahill exhausted his pursuers' fuel tank only to leave them behind after refilling his own from a reserve can in the boot. When 24- hour surveillance followed, he intimidated pursuing gardai individually by sounding his car horn whenever he passed an officer's house.
But the film's darker implication that surveillance was withdrawn only the day he was shot, suggesting possible collusion in the shooting, has been challenged by garda sources who say it ended months earlier.
And officers who saw a preview of the film said afterwards that it did them no favours. With six of his closest henchmen behind bars, he had become increasingly isolated and a prosecution was reportedly imminent.
To complicate matters the INLA also claimed the shooting of Cahill, while another major Dublin criminal rejected the IRA version, saying he knows who ordered the killing and why. The IRA linked him to the failed bomb attack on a republican social event at Dublin's Widow Scallans pub in May 1994, blamed on Portadown loyalists (Cahill had offered some of the Beit paintings to the UVF). Cahill had also fuelled republican ire by refusing to hand over part of the profits from a jewellery raid some years before.
This larger-than-life act followed him literally to the grave. Amid the grandiose mausoleums of Dublin's Mount Jerome Cemetery, as hard men placed single red roses on his coffin, came Cahill's swansong, a guitar-strumming singer performing a Diana Ross song as a lament, with its enigmatic chorus "And darling, every time you touch me I become a hero".
In the film, nominated this month for the competition section of the Cannes Film Festival and in the running for the Palme D'Or award, John Voigt, who worked with Boorman on Deliverance, plays the Cork-born inspector with whom Cahill has a long-running feud.
The criminal himself is portrayed in a reportedly stunning performance by the Irish actor Brendan Gleeson, previously a boisterous Michael Collins in The Treaty, and recently seen in I Went Down and the television drama about the Irish independence struggle.
Comic moments in The General include a droll echo of the opening of The Godfather. In Coppola's original, the elderly Don hears requests for favours. The down-at-heel Dublin version has supplicants in a snooker hall seeking gifts but getting stolen nappies from the would-be guardian of the people.
Similarly, when a comrade in crime moves to give him an embrace and kiss, Cahill pushes him away saying: "Get off that! We're not Italians."
'The General' opens on May 29.