YOU enter the theatre at 7.30 and leave just after nine, having seen three plays in a row. That sounds like very good value for money, but it may be just an irritant - depending on the plays, and the way they're done.

Many one-act plays were written and performed in the first flush of the modern Irish theatre but now, alas, are almost forgotten. ''One of the legacies of the Abbey Theatre's early years is a vast repertoire of one-act plays,'' says director John Crowley. ''These short plays sit around gathering dust, like antiques no-one seems quite sure what to do with

any more.''

Crowley dusted off three of them which he then directed as part of the current Royal Shakespeare Company season at Stratford. They are now being staged in the smallest of the RSC's three theatres, the Other Place, under the portmanteau title Shadows, and described as ''a trinity of plays''. Not one, not three, but three in one.

Part of the difficulty of staging one-act plays is one alone hardly makes a good night out - at least not in the traditional view. And if several short plays are strung together to fill out the evening, the result can be bitty and unfocused.

Crowley's solution was to choose three plays with strong common elements (including the same characteristic Irish voice), cast from the same group of actors, present them in a unified style, and set them in the same sparsely furnished space. In this way he has made links not only between J M Synge's Riders to the Sea (a tragedy) and The Shadow of the Glen (black comedy), but also with W B Yeats' Purgatory.

Death is a theme in all of them. In Riders to the Sea (seen recently in Glasgow's Citizens' Theatre), the hungry sea takes the last of a widow's sons. Death proves illusory in The Shadow, when an old man rises wrathfully from his bier too late to stop his wife making off with a sympathetic passing stranger. Death inescapable (though not his own) returns to haunt the father in Purgatory, where mysticism meets murder in the grounds of a ghostly ruined mansion house. Purgatory is the briefest of the three plays - no more than a few pages of simple but charged dialogue. Two tramp-like figures and a tree - it sounds like Beckett, but that analogy is misleading. Certainly, it's a rare and welcome chance to see one of those haunting Yeats short plays which so seldom are allowed off the page.

By a stroke of directorial legerdemain the three plays become one. No intervals break the concentration. Instead, the drowned man laid out on the kitchen table somersaults to his feet, the grieving mother and sisters scatter, the props are borne away and a new play begins. Each is separate, yet blends seamlessly with the others.

There's a high standard of acting from the Irish cast, and it will be a long time before I forget the piercing intensity of Mairead McKinley as

one of the daughters in Riders to the Sea, and her scornful assurance as the unfaithful presumed widow in The Shadow.

There's no doubt that Shadows work. It works as a rounded piece of theatre, as an emotional experience, and as an artistic unity. Like all the best drama, it's fulfilling and thought-provoking.

But perhaps it's a one-off. Could you take another three short plays, each perfect on its own, and integrate them equally well? Do you want to? Or is Shadows fortuitous, a happy inspiration best left unrepeated? I'd be interested in the attempt. Irish theatre is blessed, I dare say, but whether we might find the same rich seam in Scottish writing is more doubtful.

A faint air of old-fashioned ''am dram'', possibly justified, still hovers over our legacy of one-act plays.

But it's possible that we have writers who'd be happy to experiment with short plays, relishing the challenge of working in a demanding genre where words may be few but each one counts. Who knows what Shadows may foreshadow?