At 16 years old, her father, the actor Cyril Cusack, offered a sobering perspective on his eldest daughter's desires to follow in his footsteps.
"You might do alright in film and television, but you'll never succeed as a classical actress," he said. "You haven't got the equipment."
By the time Sinéad Cusack played Portia in the Royal Shakespeare Company's 1981 production of The Merchant of Venice, Cyril recompensed the only way he knew how.
"He never ate his words," she says. "Cyril never did. But he did give me his make-up box when I was playing Portia. I take that optimistically as a gesture of satisfaction. Maybe even praise."
Praise. A word she's been astonishingly privy to throughout an astounding career. She was praised for her early enactments at The Abbey, notably Louis MacNeice's One for The Grave when she was only 18. And she garnered critical acclaim for her turns in the West End and on Broadway in Our Lady of Sligo, Much Ado About Nothing and Tom Stoppard's Rock 'n' Roll, the last two earning Tony Award nominations.
And applause came recently with an IFTA win as Best Supporting Film Actress for her uncomfortably evocative performance in the current big-screen treatment of John Banville's The Sea.
Where adulation thrives however, doubt lingers like an undercurrent.
She says: "I have doubts every day. "Whether I can do it, whether I'm capable of doing it. Every part I take on, I think, 'This is the one where I'll be find out.'
"So often I'll feel I haven't got the tools. I haven't got the equipment, I haven't got the understanding. I'll never be able to do it. It's like you're at base camp at Mount Everest looking up, thinking, 'I won't be able to scale that.' But you trudge on."
Might this have leant in some way towards Cyril's reservations, his own experience of this crippling fear? And in turn, feed into Sinéad's early reluctance to see her youngest, Max, with husband of 36 years, Jeremy Irons, pursue the same vocation?
"Of course. As a parent, you wish your child would have an easy ride. Max is very determined, very committed, very talented. He'll do well, I think. I hope."
Her eldest, Richard Boyd Barrett, adopted in 1968 after her relationship with theatre director Vincent Dowling, is a TD for the People Before Profit/United Left Alliance in Dun Laoghaire, while her first-born with Irons, Samuel, is a photographer.
Young Max is the sole heir to the Cusack acting dynasty, enjoying Hollywood exposure with Saoirse Ronan when they starred in The Host.
"I watch Max through a haze of tears because I can't really believe my child is up there doing the things he's doing. I can't be objective about him, but I know he's good because he persuades me. And I'm his mum."
Cusack, 66, shines alongside Ciaran Hinds, Charlotte Rampling and Natascha McElhone in director Stephen Brown's adaptation of The Sea, a multi-faceted, haunting tale of tragedy and loss.
Hinds is superior as a psychologically crippled widower, struggling to piece together his being within a terrifying kaleidoscope of childhood memories after the loss of his wife, portrayed by Sinéad. As a lifelong fan of Banville, Sinead found the opportunity irresistible.
"I've read all John's books, every single one of them. I'm a devotee of his work.
As she was on stage in London, Irons collected her IFTA award for her performance in the film, which was shot in Wexford. On the night, the Oscar-winner mourned his enduring absence from the Irish film and stage industry.
Might the twosome be tempted to undertake their first collaborative project since Bertolucci's Stealing Beauty, through a local production? After all, Irons is now an honorary Corkman, officialised at a ceremony in the city in January.
"That made me laugh a lot," says the actress, who lives in Kilcoe Castle outside Macroom with Irons. "It certainly something we would fully appreciate but the previous jobs happened because the director wanted that kind of dynamic."
It's been 24 years since she last worked with her equally successful siblings, Sorcha and Niamh, in a production of Chekhov's Three Sisters at The Gate: would she like another familial collaboration?
"Absolutely," Sinead says. "Naturally there are tensions of a family dynamic on stage and where one sits in the hierarchy. And all that has to be dealt with and negotiated as sisters, on stage and off.
"In the end, it is infinitely advantageous because you have a common understanding.
"Should the opportunity arise, I'll be there like a shot."
'THE SEA' IS IN CINEMAS NOW