I'm ashamed to say that I'd never heard of Caitlin Macnamara; ashamed, because this engrossing one-woman play is an hour-long rebuke for that oversight. The wife of poet Dylan Thomas, blamed by some for feeding his self-destructive reliance on revelry and alcohol, Mike Kenny's 2004 script urges us at last to see Caitlin in her own right. Speaking directly to us as she swigs from a bottle of whisky, she tells a story of poverty, betrayal, lust and abuse – yet also of love and fellowship.

Caitlin – at least the Caitlin of this play – wouldn't want us to think her life a tragic one. There's no sense of victimhood in the script or the portrayal; her love affair with Dylan is genuinely beautiful, and it's clear she is a willing participant in much of the debauchery that follows. They had sworn to be together always, she says, and she delivered on that promise… even when it meant leaving their young baby alone at home, and joining her husband for yet another night in the pub.

You sense that Caitlin regrets that decision, and much of the play's emotional impact comes from her quiet doubts about the choices she's made. The fate of one of her children is especially startling, drawing audible gasps from the audience on the night I attended; the feeling that she's sacrificed her own talent is poignant too. The monologue is set well after Dylan's death, yet we constantly feel his presence, a force that Caitlin will never quite escape from. And while there's deep respect for Dylan the poet – and tender love for Dylan the man – we all share a growing awareness that the flaws she describes are fundamental ones.

And yet, at times, this is a very funny play. Playwright Kenny displays a deft touch in unexpected one-liners, and Kempell's timing is perfect, selling the humour without devaluing the complex topics it tackles. Caitlin's sharp-tongued assessments of people she doesn't like are often hilarious, but there's a serious point to them too – setting her relationship with Dylan in the context of his own family life, hinting at something surprisingly infantile beneath the poetic veneer.

So what went wrong in the marriage? No one thing, it seems, but a series of pressures: money, motherhood, Dylan's escalating fame and frequent trips to New York. And of course, there's the backdrop of alcoholism, a topic that's never quite discussed but is front-and-centre of our consciousness all the same. As the hour progresses, Kempell's performance perfectly embodies an almost imperceptible descent: from simple youthful exuberance into something close to madness.

And that whole-hearted, utterly convincing performance throws light on the intriguing questions posed by Kenny's script. This is no black-and-white story of a woman eclipsed by a famous man; instead, it's a nuanced and balanced portrayal, painting Caitlin as strong and independent but somehow not quite able to take control. It addresses the dark side of life with Dylan, but it also talks about the things which were exciting and good. A treat to watch – with plenty to think about after.