Celts vs. Vikings: not so merry wars; Lion of Ireland: The Legend of Brian Boru, by Morgan Llywelyn. Boston: Houghton Mifflin Co. $12.95.

"For the great Gaels of Ireland Are the men that God made mad, For all their wars are merry, And all their songs are sad." Chesterton's verse came readily to mind as I proceeded through Morgan Llywelyn's new historical novel about the Gaels, "The Lion of Ireland."

The novel celebrates Brian Boru (941-1014 A.D.), a warrior-king of the Dal Cais tribe in the Kingdom of Munster. The youngest of 12 tribal sons of tribal chieftain Cennedi, Brian was an obscure member of one of the many bands of Celts fighting without much purpose among themselves and their Norse overlords. It remained to Brian to unite the Celts and devote all the energy of his long life battling to free Christian Ireland from pagan Viking domination.

Brian's bravery and sense of mission have inspired many an Irish poem and Norse saga. His wars, while not always merry, were pursued so fervently that they have earned him more than mention in the history books. And so many husbands, brothers and sons fell under the sword or axe fighting with him, that the sad song of Dierdre of the Sorrows -- "The lions of the hill are gone, and I am left alone" -- had more than a musical significance to the bereft ladies of the time.

King Brian died at the Battle of Clontarf, too old to fight, ambushed in his tent by a fleeing Viking. His son and heir, Murrough, fell in the battle as well. But the Celts prevailed, effectively destroying forever the power of the Norsemen in Ireland. The princes descended from Brian Boru, the O'Briens, subsequently ranked as one of the chief dynastic families until the advent of the Anglo-Normans.

In this novel, Brian's lust for revenge is inspired by his childhood escape from death, when his tribe is nearly annihilated by the tribe of King Ivar from the Norse city of Limerick. Brian's mother and several brothers are slain in gory detail, and he vows to avenge them. After a brief but telling period spent in a monastery school, he succeeds his older brother as chieftain.

The brother has neither stomach for battle nor strategic imagination, but Brian has both. While in the monastery, he is exposed to history, and he puts Alexander's battle plans, Julius Caesar's close relationships with his troops and Charlemagne's leadership to work for him when he becomes a warrior.

As a result (and because he is also strong, handsome, and fearless), he becomes a charismatic leader. Legend says that when his troops attacked their enemies with the cry "Boru! Boru! Boru!" dogs howled and banshees wailed. The other Celtic tribes fell in with him and the Vikings faltered.

The lack of written records of the time, combined with scholars often at odds with each other over contradictory sources, have provided no adequate textbooks of early Irish history. Thus, Brian remains a shadowy figure, and fleshing out battles and legends with a significant picture of Brian the man requires not only a deft imagination but a faultless prose style. Too often, Llywelyn's mawkish writing gives Brian's personal life, and his encounters with nubile Druid maidens and demented princesses, the tone of True Confessions. Passion and jealousy in this book are no match for the cries and clamor of the battle scenes, where the author's most spirited and convincing passages are to be found.

"The Lion of Ireland" is not likely to garner the praise of "The Year of the French," last year's historical novel of Ireland and a winner of the National Book Award -- perhaps because tenth century Ireland is a bit too primitive and remote to capture the popular imagination as 18th century Ireland does. In addition, "The Lion of Ireland" is besieged by unpronounceable Gaelic names -- Fiacaid, Firboggs, Eochai. It reminds one of the parody by Arthur Guiterman of Chesterton's opening verse:

"For the young Gaels of Ireland Are the lads that drive me mad; For half their words need footnotes, And half their rhymes are bad."