VAN MORRISON & THE CHIEFTAINS - "Irish Heartbeat" (Mercury Records).****

American lovers of Irish music have traditionally had two choices in Irish music: folk music or Irish rock 'n' roll. But there's always been a prevailing attitude that when it comes to Irish music, rock is rock and folk is folk and never the twain should meet.But now comes the long-awaited collaboration that will make Irish music fans go crazy with delight. Take the most famous Irish folk

band in the world (the Chieftains) and add to it undoubtedly the best Irish blues rocker of all time (Van Morrison) and you get "Irish Heartbeat," one of the finest albums released this year.

It's also an exercise in cooperative creativity, with Van Morrison and Paddy Maloney combining their arranging and production skills to create an album combining Morrison's classic vocal style with traditional Irish music.

In "Irish Heartbeat," Morrison's music has more Irish flavor than ever before, and the Chieftains' music takes on a more contemporary feel.

The Chieftains have garnered worldwide acclaim for Irish traditionalism. They play ancient Irish tunes on original instruments, and they look and act the part. But for the most part, the vocal element has been missing. "Irish Heartbeat" adds that element, and their music has never sounded better.

On the other hand, Van Morrison has been a rock pioneer, especially when it comes to blues, soul, jazz and infusing Celtic elements into his music. And he developed into one of the finest vocalists around.

"Irish Heartbeat" combines the best of both worlds. And it's a better world for it.

The album is composed of eight traditional tunes with arrangements by Maloney and Morrison and two original Morrison compositions - the title track and "Celtic Ray." All are performed by the Chieftains, who with their traditional instruments layer the music with a sound as lush as the Irish countryside.

Chieftain Kevin Conneff backs Morrison on some cuts, but it's the sensational backup vocals of Mary Black, Maura O'Connell and June Boyce that are the perfect highlight to Morrison's throaty delivery and the Chieftains' light-as-air instrumentals.