GILLIAN Armstrong's second feature, "Starstruck," opening today at the Key, confirms the promise of "My Brilliant Career" in impressively different circumstances. The setting is contemporary and the genre specialized to the verge of extinction--an original movie musical comedy. The results are skillful and enjoyable enough to suggest a remarkably attractive directing talent.

"Starstruck" deals with characters who crave fame and success. They are viewed with affectionate amusement and given credit for possessing measures of performing and managerial talent sufficient to justify their brash and sometimes humiliating attempts to attract attention. "Starstruck" evokes the infectiously happy atmosphere of the old Mickey Rooney-Judy Garland musicals like "Babes in Arms" and "Babes on Broadway" in an updated pop music context.

Not that Armstrong has comparably high-powered performing talent to exploit. No one in "Starstruck" comes close to lighting up the screen with the incandescence of Rooney and Garland in their singing-and-dancing youth, but there is an engaging team at the center of the show and an entertaining troupe of comic character actors and rock musicians deployed in productive support.

As the heroine Jackie Mullens, a Sydney pubkeeper's daughter who aspires to a career as a rock vocalist of likably zany tendencies, Jo Kennedy seems a fresh, ingratiating mixture of characteristics. In fact, her natural performing rhythm is so hilariously dynamic that the movie gets a wonderful comic effect out of an episode that shows her forlornly subdued, changed by a misguided TV producer into a listless imitation of a sorrowful country and western songbird.

Jackie's ambitions are encouraged and orchestrated by her younger cousin Angus (Ross O'Donovan), an embryonic show business tycoon who is inclined to ignore his high school education while dreaming up publicity stunts designed to launch their careers. Stephen MacLean's breezy, sharp-tongued screenplay outlines a sequence of hopeful efforts that ultimately lead Jackie and Angus to a professional breakthrough, and Armstrong creates the illusion that the musical numbers erupt more or less spontaneously.

The level of musical comedy stylization fluctuates from dazzling to dubious. For example, the movie opens precariously high, with Kennedy spearheading a feverishly wacky, sensational production number set in a rock club called the Lizard Lounge, but it ought to have been reined in a bit to allow adequate introductions to the leading characters before they get lost in whirlwind musical interludes. Later, Armstrong seems to give choreographer David Atkins his head on a couple of pretexts that should have gone back to the drawing board, including a facetious water ballet for gay poolsiders, a goofy homage to Busby Berkeley that begins too abruptly.

The rock score, principally composed by members of a trio called The Swingers--the equivalent group in the movie is a quintet called The Wombats, an aspiring band that collaborates with the heroine--is charged and frequently funny. A slaphappy non sequitur of a lyric from the concluding song can be taken as an admirable summation of the movie itself: "It's the monkey in me that makes me wanna do it/ The monkey in me that makes me wanna chew it . . ." When the stray joke or musical number misfires, the effect remains forgivably silly rather than ponderous or self-destructive. The sequences may fall short of consistent musical comedy inspiration, but the swift pace and friendly tone preserve a general mood of irresistible monkeyshines.

The constant factor in Armstrong's talent as she shifts from a period romance to a modern musical comedy is probably her affectionate sense of characterization and social setting. The comfortably deteriorating working-class bar that supports the sometimes outrageous stunts of Jackie and Angus is very warmly observed, along with its set of human fixtures, notably the women who run the establishment: Jackie's widowed but still stylish and hopeful mother (Margo Lee) and her corpulent, somewhat abstracted great-aunt (Pat Evison), who observes matter-of-factly that "some of us are quite capable of talking to another world." It's a household distinguished by three generations of avid, ambitious, independent females, and someone remarks in passing, "Women have always had the push in this family."

No doubt, but there's an ironic resurgence of masculine push in little Angus, who may have a genius for getting things stirred up and accomplished. Abandoned by his parents when "his dad went fruit picking and his mum went to Jesus," Angus has grown up with Jackie, sharing an emotional rapport that causes her to describe their relationship as "Siamese cousins, actually." The heart of the movie's appeal is its sane, tolerant comic perception of their longing for recognition and success.

As far as one can see, Jackie and Angus have a lot of energy and fun to offer the rather tacky branch of show business they're attracted to. They might even spruce it up a bit. At any rate, they deserve a shot.

"Starstruck" responds to their aspirations with the same sort of amused, slightly incredulous generosity shown by a young woman questioned by newscasters about the kids' wildest publicity stunt, a tightwire act staged in downtown Sydney: "I don't know what she's up to, but I hope she gets somewhere." STARSTRUCK

Directed by Gillian Armstrong; screenplay by Stephen MacLean; choreography by David Atkins; musical director, Mark Moffatt; director of photography, Russell Boyd; produced by David Elfick and Richard Brennan for Cinecom International Films. This film is rated PG. THE CAST Jackie . . . . Jo Kennedy Angus .. . . Ross O'Donovan Pearl . . . . Margo Lee Reg . . . . Max Cullen Nana . . . . Pat Evison