THE YEAR OF THE FRENCH, the first novel of Thomas Flanagan; a professor of Irish literature and history, tells an astonishing and terrible story. It is certainly the finest historical novel by an American to appear in more than a decade.

The center of Flanagan's book is a combined French-Irish military venture, with a bright beginning and a deadly close, during a single summer in 1798. Around this Irish rebellion aginst the British he builds up a complex, brilliantly styled narrative that plays off omniscient survey against the partial views of no less than five contemporary witnesses - a Church of Ireland minister, a Catholic village schoolmaster, a youthful English aide to General Cornwallis, a solicitor member of the Society of United Irish-men, and the solicitor's English wife. Through these marvelously evoked and distinct voices the very complicated and conflicted social realities of late 18th-century Ireland come to life. Dozens of vividly conceived characters of both sexes - Protestant and Catholic fanatics, peasants and poets, landowners and militia men, the historically noted and the nameless obscure - take the stage in his erie drama.

In 1798 in counties, Wexford, Carlow and Kilkenny, thousands of country-people, commanded by gentlemen republicans belonging to the United Irish movement of Woltz Tone, and by some half-mad priests of charismatic character, fought British army regulars and well-armed bands of loyalist yeomenry. There wer frightful atrocities committed on both sides before the native insurgents gave way to superiority of arms, numbers, and organization. The next theater of rebellion was in the north, where insurgency failed quickly owing to seeds of distrust between Catholic and Presbyterian rebels. Finally, during that summer of '98, the revolutionary spotlight switched to the remote coast of North Mayo.

There, at Killala, to everyone's surprise, a French expeditionary force of one thousand soldiers under the leadership of the well-known General Humbert came ashore and headed inland. This tiny army was all that Wolfe Tone had been able to win from the French Revolutionary Directorate after years of intriguing, begging, and haranging in Paris. Should Humbert make progress and succeed in attracting a large force of native rebels to his nucleus of battle-tested veteran soldiers, then two more expeditions of French would be launched. Such was the promise, but of course the Directorate, which made something of a specialty of arousing then frustrating Irish hopes in those days, sent nobody else at all.

Humbert, after making contact with local leadership in the United Irish organization and with "White Boy" agrarian terrorists, marched to Castlebar where he defeated a large army of British regulars under General Lake. Indeed the English ran away so precipitately that the battle became known in Irish tradition as "The Castle-bar Races." After this victory, when everyone expected him to go north into the mountain fastnesses of Donegal, there to await reinforcements, he chose instead to lead his mixed force of French infantry and untrained Irish pikemen eastward into the central bog of Ireland.

Soon Lord Cornwallis, who had learned military guile from hard service in India and at Yorktown, had several blocking English armies manuvering around him. The French soldiers, who expressed a savage contempt for their rustic allies, had no idea where they were. And the Irish, most of whom had never traveled more than a few miles from their poor homes in Erris or the Nephin Beg range really didn't know what they were doing after the first few days of forced marches and starvation rations.

General Humbert's quixotic, possibly mad adventure ended at Ballinamuck - "the place of the pig" - in a dismal part of County Longford. Hordes of British cavalry surrounded his exhausted and disoriented force. The French immediately surrendered and were escorted to Dublin where they were billeted in comfortable inns before being repatriated to France. But the Irish were not allowed to surrender. Instead they were driven out onto the bog, where the cavalry and foot dragoons had a field day sabering and bayoneting them to death over a period of several hours.

As he masterfully traces the full course of this most brutal and eccentric military campaign, Flanagan avoids partisan myths while deploying his ironies, wryly, compassionately, authoritatively. In part he benefits from the recent harvest of Irish historical scholarship. But The Year of the French is itself a permanent contribution to the new, demythologized history of Ireland. Without doubt it will find a wide audience of serious readers here in the United States and in the three European countries from which the book cast of characters is drawn.