This play is brilliantly produced and performed. It is hardly uncommon these days to have a work of art whose central theme is dementia, but it is rare to see one this good. What distinguishes Lost Lear is the quality of its writing and production: the play is a carefully constructed piece of theatre with thoughtful detail and sharp drama.

As the title suggests, the play depicts the rehearsals of an adaptation of Shakespeare’s ‘King Lear’ with Joy (Venetia Bowe), playing most of the parts, and Liam (Manus Halligan), playing most of the other parts. Silently in the background are the stagehands (Clodagh O’Farrell and Em Ormonde), and Conor (Peter Daly).

Gradually we learn that Joy is a retired stage actor with dementia: ‘King Lear’ is just where her memories have taken her, and the other cast-members are all nursing-home staff whose care philosophy involves facilitating the constructed reality of her memories rather than trying to force her back into the day-to-day one of theirs.

The exception is Conor, her son and the carrier of the play’s drama. Conor struggles to adapt to this strange way of being with his mother; he was never onstage in his life and when he is finally called to deliver lines keeps messing them up.

Joy does not remember who Conor is, which breaks his heart twice over because of his past: he did not know his mother until he became a teenager. When he got in touch with her, she raised all his hopes and then rejected him, ignored him completely. Conor was never a part of Joy’s world and has carried the hurt and resentment of this through his adult life. For him, adapting to Joy’s disease simply repeats the pattern of their whole relationship in which it is Conor who must find a space, sometimes despite Joy’s best efforts, in her busy and irresponsible existence.

I have just given away one of the show’s big twists – it is coy about the real circumstances of its ‘Lear’ rehearsals for a long time – but I have not given away everything because it continues to develop Joy and Conor’s relationship with great skill. Their perspectives are developed fully and without judgement, and then by circumstance forced to occupy the stage together. The drama comes naturally. The performance does not choose a side and audiences can read into it whatever they like. Maybe it is not a coincidence that a show loosely based on one of Shakespeare’s tragedies exhibits to such a degree one of his most important dramatic skills: the ability to ask questions without answering them.

And it also points towards how, despite its marketing, this is not really a play ‘about’ dementia as such, but one in which dementia figures with its effect on one person’s life and relationships with other people. It is no more about dementia than ‘Raglan Road’ is about a street in Dublin.

The acting is another highlight. In a high-quality cast, far and away the best performer is Manus Halligan as Liam: he moves seamlessly between roles.

The acting is another highlight. In a high-quality cast, far and away the best performer is Manus Halligan as Liam: he moves seamlessly between roles corresponding to the different lived realities onstage, but every time he is completely believable. He is worth looking out for in something else. Venetia Bowe is a very convincing Joy, but sometimes the emotional changes of gear are a little jerky. Peter Daly’s Conor comes into his own as the show progresses, but at the start Conor’s nervousness is perhaps a little overplayed. Overall, the quality of each surpasses what you would normally see on one of the big stages in Dublin.

This is one of those shows where elements of the stage and set design played a huge part in telling the story. Sometimes actors speak into a phone-camera on a stand stage-right which projects their faces Wizard-of-Oz-style onto a translucent curtain in front of the stage, behind which some other action is visible to the audience. Sometimes the actors say their lines in front of the curtain: this becomes important where the relatively young Venetia Bowe, who is obviously playing an old woman in Joy, speaks while behind the curtain Joy herself is represented by a puppet.

Most such moments are effective as storytelling methods. The weakest one is the last, in which, sitting alone onstage, the puppet Joy’s brain lights up and the back of the stage shows a kind of intergalactic starry visual sequence with space-music as the lights go down and the curtain falls. The moment’s final emphasis on Joy’s dementia is understandable, if a little off-piste.

The biggest problem is the scene’s diminution of Joy combined with its mystical mood of wonder: its ‘We are all made of stardust’ attitude. This is cheap even when Brian Cox does it on the BBC, because it is a completely manufactured sense of wonderment. What is wonderful, apparently, is the paradox that we human beings are simply a collection of chemical and physical processes whose workings are complex but essentially banal. The banality of the stars (as well as their disappearance from the modern night sky) is a central consequence of Western scientific metaphysics and engineering. But that means for artists they are not allowed to be so easily wonderful anymore, since they are nothing but instances of the mute universe that we ennoble by speaking on its behalf.

The only wonderful and remarkable things are those on t Earth: we who ennoble the stars with our awareness and awe, as well as every plant and animal that does the same. Mystically the stars have nothing to give us, it is we who have everything them. There is no sense in which a person (in this case, Joy) is reducible to a pile of neural networks any more than they are reducible to their heartbeats or even something social like their nationality or which school they went to. Like neural networks, all these relate to actual things and yet for a person they are all abstractions. The neural networks are an abstraction and Joy is not.

But the ending was a small mark against this performance of such thorough professionalism and humane sensitivity. If any schools are teaching ‘King Lear’, they should take students to this.

Lost Lear is running in the Project Arts Centre until 8 October as part of the Dublin Theatre Festival.