October 1999, and The Oberver newspaper's Life magazine decides, one Sunday, to celebrate the new wave of British play-writing, currently the envy of Europe. The only problem is that of the six main playwrights to receive a mention, three - Martin McDonagh, the author of The Beauty Queen of Leenane, Conor McPherson, who wrote The Weir, and Sebastian Barry, whose Steward of Christendom was an immense hit in London four years ago - are not British, but Irish; which serves to remind us of at least two things.

First, that the self-absorbed world that is the London arts scene still finds it as difficult as ever to recognise the real cultural and national diversity of these islands; to appreciate without appropriating.

And second, that now, as in almost every generation since Sheridan, the contribution of Irish writers to English-language theatre is so immense, relative to the size of the country, that people have almost given up trying to analyse it, and tend to take it for granted, as something God-given.

For the first thing to recognise, in thinking about Scottish and Irish theatre, is that we are not considering like with like.

Irish theatre is a mighty thing, an art-form as close to the heart of the nation as its poetry and its music, one that played a significant part in the struggle for national freedom at the end of the 19th century and the beginning of the 20th, and that still looks to the Abbey Theatre in Dublin, home of Ireland's National Theatre Society, as a key focal point of national life.

From Wilde and Shaw 100 years ago, through O'Casey and Synge, to the stunning post-war genius of Sam Beckett, writers born and bred in Ireland have helped to shape English-speaking theatre across the world in this century; without them, the standard repertoire would be devastated, robbed of half its strength and brilliance.

And the present generation of Irish playwrights - not only McDonagh, McPherson and Barry, but Tom Murphy of The Patriot Game and The Wake, Marina Carr of Portia Coghlan and The Bog of Cats, Dermot Bolger of Lament for Arthur Cleary, Mark O'Rowe of Howie the Rookie, and the great Frank McGuinness, author of Observe the Sons of Ulster Marching towards the Somme - are the clear inheritors of that great text-driven tradition, mediated by superb acting.

According to the writer and critic Fintan O'Toole, Ireland is remarkable for having passed from tradition to post-modernity, in the second half of this century, without ever really becoming "modern"; and that is as true in Irish theatre as it is elsewhere.

Scottish theatre, by comparison, is a more tentative and hesitant affair, and traditionally much less central to the national life. The reasons for this marginalisation are complex, and certainly have something to do with the disruption of Scotland's indigenous theatre tradition, particularly at the court and literary level, by the Union of Crowns in 1603 (which took the court to London), and by the Presbyterian Reformation of the late 16th century, which strongly disapproved of colourful and sensual public displays of any kind.

But whatever the causes, it is fair to say that over three centuries of Union with England, "legitimate" theatre came to be seen by most Scots (and by most theatre managements in Britain, who tended to treat Scotland as part of their "provincial" touring circuit) as essentially a London-based phenomenon, dominated by English repertoire and English companies; and that it's only in the last two generations, since Scottish society became more self-consciously pluralistic and public subsidy for theatre became available, that Scotland has begun to develop a strong, distinctive, continuous modern theatre culture, often combining elements of conventional text-based theatre with forms borrowed from strands of popular entertainment - notably comedy and variety - that always remained Scottish in tone.

OF course, this is not the whole story of Scottish theatre in this century. In the 1940's, many writers involved in the powerful amateur drama movement in Glasgow wrote big, conventionally-structured working class dramas - The Gorbals Story, In Time of Strife, Ena Lamont Stewart's famous Men should Weep - very much in the style of Sean O'Casey, trying to give working-class experience a place on the Scottish stage.

Around the same time, writers like Robert McLellan and Robert Kemp worked hard to create conventional large-scale drama in the Scots tongue. But their impact on the professional theatre scene was limited until the key decade of the 1970s, when John McGrath's 7:84 Scotland Company - particularly with their greatest and best-known ceilidh show, The Cheviot, The Stag, and The Black, Black Oil - seemed to liberate Scottish theatre in a completely new way by combining popular forms like ceilidh and variety entertainment with a strong dramatic and political argument, and building a new bridge between "legitimate" theatre and grass-roots Scottish culture.

In 1980, McGrath used his 7:84 clout to begin a series of revivals of the lost working class plays of the 1930s and 1940s; and a year later, the short-lived but influential Scottish Theatre Company was launched.