“There is so much that none of you will understand about me or where I come from”, Johnny Byrne sneers. And he is absolutely right. I am so grateful that the unremittingly bleak Glasgow gang-world that Byrne inhabits is so alien to me. Yet despite the stark differences between the shamefully deprived Gorbals area of Glasgow in which the play is set, and the comfortable West London location of the Finborough Theatre that houses it, we as an audience are fully absorbed in The Hard Man. Indeed, we are pummelled into submission, akin to one of Byrne’s victims, almost suffocated by the near-to unbearable intensity sustained over two-and-a-bit hours.

And boy does writer Jimmy Boyle know his stuff. Sentenced to life imprisonment for the murder of another gangland figure in 1967 (a crime he denies, I’m very quick to add), Boyle’s words clang with gritty authenticity, aided by co-writer Tom McGrath, who went to meet Boyle in prison to help frame and structure the piece. Their work can be seen as a damning critique of a society that allows this to happen, and of children who grow up in dire urban deprivation and become self-fulfilling prophecies. Byrne is “destined for Borstal”, according to his teacher.

Byrne and his cronies Bandit and Slugger live in an environment where violence “is an art form. Practised in and of itself”. All these characters look pasty and unhealthy, more at home slicing faces than fruit. Yet, who is to blame for Byrne and co’s horrifying crimes? The individual, or the society and communities that raised them? Byrne’s first prison sentence is described as “like going to university” – the closest thing to university that these characters and the individuals they are based on will ever get to see – and “like a top-level conference”.

The Hard Man is claustrophobically brutal. There is less light in this play than manages to penetrate through the grimy pub windows. The audience sit there, backs hunched, fists clenched, heart in mouth. Never has an interval felt so necessary. I ran into the cold March air just for a reprieve, to breath and escape. The production is technically superb, but considering its content I cannot be the only one that willed for an earlier finish. With this in mind, The Hard Man should perhaps be shortened by a good 15 minutes at each end. Director Mark Dominy is perhaps too sentimental with the text: some bizarre asides to the audience could be cut, as they appear to add to the length but not to the plot. Additionally, there are too many characters (24 in all); lose the gossiping housewives and the old prison lag and you have a slicker piece.

That said, the performances are mesmeric. Sarah Waddell provides some much, much-needed relief as the gutsy prostitute Didi, Ross F Sutherland is thoroughly deplorable as bent cop Paisley, and Adam Harley displays fine acting talents as both the menacing Bandit in the first half and the understated Johnstone in the second. Martin Docherty as Byrne, meanwhile, is flawless. He twitches about the space, totally unpredictable to the audience. He is harrowing and traumatic to watch: we despise him, we pity him, and ultimately we are entranced by him. Credit also goes to Ross Dunsmore, Jack McMillan and Ruth Milne. This is a strong cast.

The Hard Man is a stressful and unnerving experience. It rams home the hard truths about life not very far away from ours, and is a hard watch. Yet sometimes theatre needs to be.

The Hard man is playing at the Finborough Theatre until 18 March. For more information and tickets, see the Finborough Theatre website.