In the dark and bright of the 1960s - which year, exactly, I don't know - my mother one evening put me in the audience at the Abbey, as she sometimes did, to watch the production of the moment, Galileo, Brecht's epic play, writes  Sebastian Barry

In the dark and bright of the 1960s - which year, exactly, I don't know - my mother one evening put me in the audience at the Abbey, as she sometimes did, to watch the production of the moment, Galileo, Brecht's epic play. I do not remember if she left me alone or sat with me. Oddly enough, I can still sense and see myself there, in the bluish cocoon when the lights go down, still gain an intimation of that strange one-sided talk that grew between me, the starer, and the performer.

The performer in question was an actor I knew as a member of the company, but backstage, in the way the small son of an actress gets to know those vanished troupes - sometimes slightly gloomy presences lurking in their dressing rooms, sometimes resplendent personalities, all greetings and smiles and, it seemed, feathers and flounce, like my mother's especial friend Angela Newman.

I had no idea then of what theatre was in a public sense; I judged all plays equally, as equal miracles. It was a family affair, after all. I had watched as a younger child a performance of Yeats's Kathleen Ni Houlihan (but did Lady Gregory write that?) and been scared by the old woman and the ghostly last line about her, and been astonished and disconcerted to realise at the end that it was my own mother had played the part, giving rise to an intimation that has not really left me, that the stage contains mysteries and transformations beyond greasepaint and explanation.

That night of Galileo there was something extra in the air, some fierce focus and extrasensory delight, some dimension of grandeur and grace, a sort of easy willingness in the audience, faces pushed forward, hearts beating loud and fast.

Mick Hennessy was all majesty and might in the part. He assailed the mountain that Brecht had supplied. He showed to us with fiery clarity a truth-seeing man condemned by the narrow fears of his time. I know now Mick Hennessy must have listened with scared and almost noble intent to the words of his director (the very great Tomas MacAnna, as it happens, responsible for so many of the glories of those ill-remembered days).

I did not need to be told, child though I was, that here was a mere man acting to the last limits of his abilities, and that in nearly bursting the bounds of himself, he was proving himself before God and Dublin to be an actor of the first degree. It was even better to hear that he had been a toiler in less distinguished parts for many years (according to my mother) and that no one had expected this show of divine strength from him onstage. It was something beyond his grasp that he grasped with the still fever of a mortal person with his nose against the window of heaven.

This sense of an impossible role, and an actor rising against all the odds to meet it, in retrospect provided me with a secret reason to write plays - not necessarily for the explication of a family, or even a country, but in the first instance to see an actor go beyond the safe borders of his farm and strike out across that solemn and often terrifying terrain: the badlands, the flowering wasteland, of artistic imperatives fulfilled.

Hinterland by Sebastian Barry opens at the Abbey Theatre (01-8787222), Dublin, on February 1st, with previews from Wednesday