New history carves revealing path through Ireland's tangled landscape; The Irish in Ireland, by Constantine FitzGibbon. New York: Norton. 328 pp. $19. 95.

A minor Irish writer named Thomas Kelly once described his native country as ''an unimaginable chaos of races, religions, ideas, appetites, and provincialisms.''

As anyone familiar with the twists and turns of Irish history can testify, it is an apt description. The troubled mix, born of a long history of conquest and usurpation, of Celts, Vikings, Normans, High-Church English, Low-Church Scots, Roman Catholic peasants, and Protestant landlords has made the question asked by Shakespeare's Irishman in ''Henry V,'' Captain MacMorris - ''What ish (sic) my nation?'' - all but impossible to answer.

Constantine FitzGibbon's ''The Irish in Ireland'' tracks this tangled terrain from the point of view of political and social history. And it carves a revealing path through the complex landscape of Irish affairs.

FitzGibbon, an American who has lived in Ireland for 20 years and whose voluminous canon of fiction and nonfiction includes several books on Ireland, is not a professional historian, nor does he claim in this book to be writing an Irish history.

Instead, he describes ''The Irish in Ireland'' as ''a book about the Irish, about their own ways of doing and of being, of perception and belief, about what distinguishes Irish men and women and even children from their fellow humans.''

Despite this disclaimer, the book is, in large part, a history, tracing the Irish people back to the pre-Celtic Stone Age and concluding with some observations on the Ireland of today.

FitzGibbon brings a wide-ranging mind to his task and, perhaps because he is not lost amid the trees of academic specialization, is capable at times of drawing connections and deducing effects not often found in more narrowly focused histories: the striking parallels between Ireland and the United States in terms of their relationships with England in the 18th century, for example, or the way the English Reformation, by taking over the episcopal system from the Church of Rome, imposed a new, alien structure on the largely monastic Irish Catholic Church, thereby cementing the link between patriotism and Catholicism in Ireland.

Yet, FitzGibbon's attempt to write a book ''about the Irish'' takes the form chiefly of sociological generalizations wedged into his historical narrative. Some of these, such as ''The Irish run to extremes . . . they either work very hard or almost not at all,'' border dangerously on cliche. Furthermore, it is hard to imagine an ideal audience for the book; much of what FitzGibbon has to say would hardly be news to a historian, and yet the book is far too learned to appeal to the general reader.

As the sectarian violence in contemporary Northern Ireland so painfully demonstrates, there are, apparently, many different ways to answer Captain MacMorris's question of ''What ish my nation?''

This book is to be recommended if for no other reason than for its attempt to discover some possible answers to that question, or at least to understand it better - to make some sense, that is, out of the unimaginable chaos that is Ireland.