Michael is an 18-year-old from Dublin, living with his grandfather because his dad is in prison. Already on probation for riding in a stolen car, naive Michael is pressured into “moving” a bag of drugs for older, scarier acquaintances. It lands him a three-month sentence that is the substance of this terrific movie by Irish film-maker Frank Berry. It’s a prison film and a social-realist picture of the Loachian school – fierce, unsentimental, engrossing.

Considering how ubiquitous crime is in the movies it is surprising how rare the prison procedural is on screen, and how horribly gripping: the judge, the lawyers, the solicitors, the warders, the inevitable “strip” scene where the new intake are relieved of their clothes and dignity, and finally most terrifyingly of all, the yard and the real authority – other prisoners.

Dafhyd Flynn plays Michael, his open, boyish face hardening and sharpening like a weapon with every day spent inside. Lalor Roddy plays his grizzled and care-worn grandfather Francis, and Roddy shows how the old man is overwhelmed with grief at the thought of his son and grandson being in prison and also crushed by a reality of which poor Michael is still unaware. His sentence will continue on the outside.

Inevitably, Michael is confronted in jail with the key questions: when should you abandon your initial determination to keep your head down and instead violently stand up for yourself in the face of bullying? But how do you repay the debt to the hard men under whose protection this self-assertion will have to happen?

We are introduced to the abusive and dysfunctional world of prison gangs and the ritual of “holding” things for tough guys higher up the food chain: drugs, money, mobile phones. This is ostensibly so they won’t be found in a search, but is really to create a network of cowed responsibility, indebtedness and subservience. The self-replicating system of fear is eloquently depicted here.