A strange but reassuring ouroboros effect is in place in Ross Killeen’s documentary about dementia. One of the things that ailing Helena is able to recall is her son’s insistence as a child on embracing creative thinking. Now a grown man and operating as a street artist under the moniker Asbestos, art is the means by which he seeks to grasp her identity and come to terms with her disintegration. It is a labour that encircles her, spills out on to the streets of Dublin and colours the fabric of this crisp, sensitive film.

Helena no longer remembers what day of the week it is, or the month. Her husband tries to reignite her synapses by consulting her on crosswords. The film is drowning in mnemonic fragments: archive footage of family time and old holidays, and street scenes in which Helena’s strolling figure has been scrawled over. But if all this is flagging up memory’s fallibility, it doesn’t stop Asbestos trying to make sense of the inner world. His surrealist murals amplify his preoccupation with identity, and he comes up with a striking gambit to address his personal grief: an exhibition of chalk-on-blackboard drawings of his family that he invites the public to erase or draw over.

Helena – a worrier before the dementia – ironically appears happier living purely in the present. She laughs every time she sees a picture of her son. And as Asbestos adds them to his project, an unexpected calm settles over the documentary, with his mother’s tautological narration of past times working like a lullaby. The moment of letting go is elegiac rather than traumatic; hung around town, the drawings are blotted by the incoming sea, flecked into oblivion by the rain, and given loving annotations.

Who knows if art really can hold back the tide of personal annihilation – but it seems to work at least once here.

• Don’t Forget to Remember is in Irish cinemas from 6 September and Northern Irish cinemas from 7 September