With Baskervillian barking in the background and devil-tarot cards flickering across the screen, Lady Macbeth's cadaverous face curls into a nefarious grin. After slathering vermilion lipstick on his stubbled mouth, Lady Macbeth - or rather, "Ladyboy" - and a cavalcade of droog-like witches get together for a spot of skull-duggery, cellblock C-style.

Not classic Shakespeare, for sure. This is Mickey B, the Educational Shakespeare Company's adaptation of the jet-black tragedy Macbeth, shot in HMP Maghaberry in Northern Ireland. Twenty-four of the 25-strong cast are still behind bars - and probably will be for a long time. In this version of Macbeth, usurpations are carried out with shanks, paid for with packs of Golden Virginia tobacco and the poison of choice is LSD, not hemlock.

Maghaberry is a maximum security facility and a lot of the inmates are lifers, so there was no way to film outside the prison. That makes the hour-long feature claustrophobic and, at times, uncomfortable viewing. The jail's brickwork and joinery rooms were transformed into a monochrome set over two weekends and its construction necessitated co-operation between prisoners and prison officers. The scene in which King Duncan is found slain in his bed takes place in a replica of one Maghaberry's 745 single cells; 13x7ft with an open top to allow high-angle shots.

There was a frisson of disgruntlement in the Northern Irish press about the close-to-the-bone nature of the project, some of which centred around the decision to film one of the more macabre of Shakespeare plays. But Mickey B director Tom Magill (who is also ESC's artistic director) believes the logic behind producing a film based on Macbeth - the tension of which resonated strongly with the detainees, many of whom experienced brutality on the streets of Belfast - is that it helps on the long road to rehabilitation. "Look at it like this," he says. "We're dealing with non-conformed prisoners, with people from both sides of the sectarian divide, loyalist and republicans. There are people here with no qualifications whatsoever. Here, they can be awarded with educational active citizenship awards."

The notoriety of the film was exacerbated when news broke that many of the cast were incarcerated for links to gangs and terrorist cells in the North, with some tabloid denouncing the project for making film stars out of criminals. But the legal obstacles involved in getting the film released in the UK make any potential glorification a moot point. Prison regulations state that there must be a three-year delay - or "lag time" - before any artwork involving prisoners can appear in public, even if no money is being earned. There is a "lag-time" in force with Mickey B, meaning it will not be available for public viewing until 2010 at the earliest.

But for the only cast member to have left Maghaberry, the press criticism missed the point of the project. Speaking at a screening of another prisoner film at the Royal Festival Hall, Sam McClane said: "This is the most important thing I've ever done. Jail can brutalise people. You can feel hopeless. This project meant something to everyone inside. For someone to say that we shouldn't be allowed to help ourselves in that way - it's crazy."

The films-in-jail operation of the ESC is one a growing number of government-backed initiatives that focus on rehabilitation through the arts, the products of which are becoming ever more popular. Along with organisations like the Koestler Arts Trust, which holds an annual prize for prisoner artwork and literature, and Billy Bragg's Jail Guitar Doors, ESC's statement of purpose is to dispel the stigma that, no matter what, "prisoner" is an indelible identity. With further projects in the pipeline, ESC is endeavoring to imbue those detained at Her Majesty's Service with the simple notion - an actor is always an actor.

• You can see the trailer for Mickey B at tinyurl.com/9odw2w