LEAVE IT to the Irish and Scottish branches of rock and roll to come up with the "Big Music" theory. That happens to be the title of Mike Scott's breakthrough song for his Waterboys, but it's also an apt description of his approach -- and that of U2, Big Country and half a dozen others who invest their music with a pulsing idealism and serious thinking.

On the Waterboys' third album, "This Is the Sea," Scott's epic sound swirls with Spectorish sonic depth and such detail as trumpet and saxophone ornamentation and violin cushions. He has the skill of a poet, the passion of a rocker, with influences as disparate as William Blake and Van Morrison.

Scott goes gladly into the mystic, whether on love songs such as "Trumpets" and "The Pan Within" ("Nights like these were born to be sanctified by you and me / Lovers, thieves, fools and pretenders / And all we gotta do is surrender") or "Don't Bang the Drum," an exploration of the distance between the probable and the possible that prods the listener to work for the latter.

The main difference between the Waterboys and fellow Scots Big Country is that Scott's grandeur is more lyrical than musical. In "The Whole of the Moon," he pits information against consciousness ("I spoke about wings / You just flew / I wondered, I guessed and I tried / You just knew . . . I saw the crescent / You saw the whole of the moon."). In "Old England," Scott looks with melancholy heart at the sunset of the empire ("Evening has fallen / The swans are singing / The last of Sunday's bells is ringing / The wind in the trees is sighing / And Old England is dying.").

Whether it's the paranoid fantasy "Be My Enemy" or the urgent "Medicine Bow," Scott always cultivates his lyrics like a farmer would his fields. "This is the sacred ground with a power flowing through," Scott sings at one point. One suspects he's singing not just about the land, but about the music as well.

THE WATERBOYS -- "This Is the Sea" (Island 90457-1); appearing Sunday night at 9 at the Bayou.