IT MAY be a quarter of a century since the McGarrigle sisters released their first album and enjoyed their highest public profile, but it has always been a source of some consternation that their considerable song-writing expertise has only been sporadically recognised.

Suffice to say, Kate and Anna are not generally thought of in their rightful place in the echelon of songwriters. Rarely are they mentioned in the same breath as Neil Young, Joni Mitchell, or Leonard Cohen, and there has never been a Kate and Anna McGarrigle Unplugged on MTV.

For while this may be a miscarriage of justice, their show does much to explain their failure fully to connect with the mainstream. Musically, it is understated, and the lyrics and between-song conversation point to a healthy amount of irreverence and self-depreciation.

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''We don't play live very often,'' says Kate by way of introduction. ''But in the last year we have played at a French festival, a blues festival, and now a Celtic festival. Maybe we are a little schizophrenic.''

Even so, top-drawer songwriting manages to cross boundaries: The Brel-like Ce Matin sits comfortably next to the blues of Somebody On Your Bond and their more trademark piano-based songs like I Eat Dinner and Heart Like a Wheel. Short of some of the instrumentation, the Celtic connection is a convenient, rather than glaringly obvious, one.

The lack of regular performance may also take a slight toll: though the mood is endearingly informal, it could also disguise under-rehearsal.

Equally, songs like Heartbeats Accelerating and Going Back to Harlan have been surpassed by their interpreters, Linda Ronstadt and Emmylou Harris, but this has never been used as a criticism of the likes of Jimmy Webb or Burt Bacharach. At their best, Kate and Anna McGarrigle are definitely in the same league.