The clear-eyed, chastened lasses who are sentenced to live in a convent from hell in The Magdalene Sisters are treated as sinners, but, as the movie makes frighteningly apparent, it didn't take a lot to sin in the Ireland of 1964. One of the young women, the shy and angelic Patricia (Dorothy Duffy), has a baby out of wedlock, but the only ''crime'' committed by Margaret (Anne-Marie Duff) is that she was raped. Bernadette (Nora-Jane Noone), a dark beauty with a proud sensual stare, is condemned for flirting with the boys at recess. Tossed out, in priggish disgust, by their families, these three arrive at one of the Magdalene Asylums, a stately edifice built of old stone that sits all too placidly in the emerald countryside.
Once there, they are plunged into a life of slave labor and ritual abuse, presided over by nuns who take special delight in mocking their charges for the sensuality they deny in themselves. The atmosphere of deprivation is so strict that the inmates aren't even allowed to talk to one another as they eat their miserable porridge or scrub away all day in the laundry room, shoving piles of clothes into boiling water that, presumably, is meant to sear the stains of their own flesh. Yet they talk anyway, rebelling with quiet rage.
There's no getting around that ''The Magdalene Sisters,'' in form, is a kind of high-toned women's-prison picture. Written and directed by the Scottish actor Peter Mullan (''My Name Is Joe''), the movie grasps the drama of cruelty, of fanatic puritanism turned into luridly repressive shock theater. Yet the movie is also a fearless and powerful exposé: The Magdalene Asylums really existed, and Mullan has captured their pious horror with a humanity that burns away any hint of exploitation. These girls may have been subjected to torments worthy of a fascist regime, but Mullan, an inspired director of actors, undercuts the brutality with his disarmingly tender and intimate gaze. The brilliant performance of Geraldine McEwan as the wardenlike Sister Bridget, with her twinkly mean eyes and singsong sadism, is a profound study of how, in this particular Irish Catholic cosmos, sexuality could be regarded as a thought crime. ''The Magdalene Sisters'' is the rare movie that turns cruelty into art.