Actress tells of life with the Royal Shakespeare Company

''I believe acting is the shy man's revenge,'' says Sinead Cusack, a bright star of the Royal Shakespeare Company's current American visit. We're in her dressing room at the Gershwin Theatre, about an hour before she goes on in tonight's ''Much Ado About Nothing.'' It alternates with ''Cyrano de Bergerac'' in the troupe's Broadway run, which ends Jan. 19, whereupon the RSC will travel to the Kennedy Center in Washington, D.C., for a month-long engagement.

Miss Cusack plays Beatrice in the Shakespeare comedy and Roxane in Rostand's epic. She always gets a touch of stage fright before the curtain rises, but you'd never know it from the confidence of her strong, breathy voice.

Why call her profession ''the shy man's revenge''? Cusack explains her theory.

''You get a chance to don the characteristics and clothes of other people, who do things that are brave and dreadful and heartwarming and foul,'' she says, with both glee and trepidation in her tone. ''You have opportunities to show every dark and deep aspect of your personality -ones you'd never dream of showing amongst your friends or even lovers.

''And you're hidden,'' she adds after a pause. ''Because it's only acting, isn't it? You can make adventurous leaps you might never have the opportunity to do in life, because it's really not you - or you pretend it isn't. You examine areas of life and personality that others may blinker themselves to, that are uncomfortable and difficult. You have opportunities to broaden your vision . . . .''

Then, with a sudden grin, she catches her own words up. ''But that's all very grandiose,'' she says in a bright, self-mocking way. ''Probably I just like it because they clap when I go out there!

''Maybe I'm only justifying the fact that I'm an egomaniac, a megalomaniac, and need people to reassure

me all the time that I'm wonderful,'' she goes on. ''I'm sure that's also a heavy motive for a lot of actors. We all feel inadequate, and most actors I know are the most inadequate human beings you tend to come across. Up there onstage you're reassured that everything's all right and you are actually loved . . . .''

Sinead - that's pronounced ''shin-ed'' - got her stage ambitions early. She knew the theater was for her by age 12, partly because she comes from a long acting tradition. Her father is Cyril Cusack, renowned on stage and screen alike. Her mother was also a performer, and two grandparents were ''strolling players'' who toured the English and Irish provinces.

Not that her family steered her toward a stage career. Her father remembered the instability of his own childhood, when ''he'd knock at the door of a different school every week,'' and determined to give his own children a more secure life. ''So we were academically prepared for life, very heavily,'' Sinead recalls. ''My father wanted me to be an academician.''

But her ideas were different. At just over 17 she entered University College, Dublin, and at the same time she auditioned for the Abbey Theatre, the Irish national stage company. She had no formal training, but sailed through on native talent and enthusiasm - which, she learned, will carry you just so far.

''I did my university and acting careers together for two years,'' Cusack recalls, ''and then I was thrown out of the Abbey because they said I couldn't be heard beyond the first six rows. I had an instrument that was completely untrained, and I couldn't make it do for me what the native talent needed. I couldn't translate the talent for an audience because I didn't have a voice, a body that moved to project that inner knowledge.''

It was a sudden drop from the heights to the depths, but Cusack was unfazed. ''I went to England to seek my fortune,'' she says. ''And because I had this round face and blue eyes and long blond hair, I got cast very quickly in television and films. That wasn't what I intended to do, ever, but I got sidetracked for many years.''

It's a measure of Sinead's seriousness about her art that she didn't revel in her screen success. She kept chafing to go back onstage - even though the stage had beaten her once, and a return to the theater would mean minor work in out-of-the-way venues. In fact, she welcomed the idea of rigorous training away from the major leagues. ''I wanted . . . to work in the provinces and make up for my lack of training,'' she insists. ''My ambition never changed.''

One of her strategies was to keep asking the Royal Shakespeare Company for a job, knowing it was ''a first-class teaching company'' and not just a busy performing troupe. But those doors didn't open easily. ''They turned me down year after year,'' Cusack recalls. ''Once they took me as a replacement for Judi Dench - she's probably the greatest living actress in England - but I suffered by comparison, so they threw me out again.''

It's another measure of Sinead's tenacity that she didn't waver after being thrown out of two leading European theaters. ''I should have given up,'' she says cheerfully, ''but not me!'' The breakthrough came when her boyfriend, a handsome actor named Jeremy Irons, was appearing in an RSC production with two other cronies. The leading lady decided to leave the cast, and Sinead's friends proposed her for the role. ''I had seen it seven or eight times and I knew all the lines,'' she says. ''And by then I had finally learned enough to get away with it.''

Thus she entered the RSC for good. ''Once I was in I was like a leech!'' she says with a grin. ''And the parts got better and better, and my work got better and better.''

Some things in Sinead's life have changed since those days. For one, she's now married to Irons, and spends much of her time tending Sam, their six-year-old son. One challenge here is giving the youngster security without becoming rigid about his upbringing, as she feels some actors (including her father) do with their children.

''We're operating from the viewpoint that a loving background is probably more important than a very secure geographical background,'' she says. ''My father's concern was to root us in every way, but Sam has traveled since he was five years old. He's a terrifically adaptable child as a result.'' He also loves Shakespeare, whose plays he's been attending since age 3.

Other things are the same for Sinead as they were years ago, including her lack of formal theater training. She has mixed feelings about this.

''I feel I could have been enormously helped by some measure of formal training,'' she admits. ''As a young actress, my limitations had to do mainly with my voice and with handling classical texts. I simply couldn't do it, and I was bad at it, even though I did seem to have a gift and a talent for communicating on television or in a smaller space. The difficulty of handling formal texts would have been ironed out of me in drama school.''

Since her stage experience has been mostly classical so far, it's ironic that Miss Cusack was intimidated by the classics in her early years, even though ''the worst possible evil for an actress is to be frightened by texts.'' What was the problem? ''I was reverential,'' she replies, ''as most of us are about Shakespeare. Because I'd been taught badly at school, or maybe just because I'm an Irish peasant, I felt you had to be serious about him, that you had to never smile while delivering a line.

''But that's a load of nonsense!'' she goes on. ''Of course he's the greatest living playwright, as I like to call him. But he's also the actor's friend. He gives you texts of such depth and complexity and interest that you can never be bored.''

Cusack has now been performing Shakespeare ''seriously'' for six years, and still finds ''60 to 600 to 6,000 ways of reading each text.'' She loves the Bard because ''he can be approached from so many angles. And every night there's another discovery - some nugget or little gem that makes the play glow anew.''

How would she feel if her son continued the family tradition by going onstage? ''I hope he doesn't,'' Sinead says, offering another glimpse into her life as an actress and the difficulties she's gone through. ''We (performers) put ourselves up to be shot down. We suffer a lot of humiliation and rejection, especially in the early years. We have to put a wall around ourselves to protect against that attack - rejection by critics, audiences, directors, whatever - which never stops, even when you've reached your mid-70s like my dad.

''There are great rewards,'' she adds. ''I don't mean financial rewards, which are only for a few, but rewards for the spirit and ego, which are marvelous when they happen. Still, it's not an easy or gentle life. There's lots of difficulty and pain involved that I wish my kid wouldn't have to go through.''

Concerning her future, Cusack springs a surprise: She's leaving the RSC early next year. ''There's always a danger of complacency if you remain with one area of life or work too long,'' she explains. ''I've been lucky enough to achieve an ambition I had since I was very young, but if you don't have new challenges you grow old quickly and get bored. I've met the RSC challenge and I've met it well. Now it's time for new ones.'' She isn't sure what form they'll take, but hopes her future will include more contemporary work by living playwrights.

''The wonder of theater to me - what gets me most excited - is that through the communication of pure thought, you can project to other people sitting out there,'' says the actress with gusto. ''I don't understand how it happens . . . but a lot of very exciting things can happen through that intimacy we have as actors on the stage, when we use our eyes to look at each other and our bodies to respond to each other.''