Tread a stage for the guts of 30 years and there are few roles written that'll throw you. Then one day you're presented with a character so furiously described - alcoholic, dying of cancer, flailing under the debris of a collapsed life - that the nerve ends tingle and suddenly it seems just like the first time.
"I stood shaking in the wings at Oxford when we previewed," says Sinead Cusack of her first night in Our Lady of Sligo "and I thought, I can't do this, I just can't go on, I know it's only rock 'n' roll but I can't do it. And then I went out and of course I did it."
Sebastian Barry's new play, like his reputation-making The Stew- ard of Christendom, ladles from the murky pool of personal family history and sketches a fiercely-poetic narrative around the life and demise of his grandmother, Mai O'Hara. It is, he says, about a traveller to the heart of human disaster, a traveller who doesn't come back.
For Cusack, sitting in a west Cork hotel now, skulling a mug of strong coffee and smoking Silk Cuts, it was a role that moved into her life and promptly took over.
"It has been more absorbing and over-taking than anything I've ever done before. I had to start working on it six months before rehearsals so she almost became second nature to me, I felt that I came to know her terribly well."
Mai must have immediately appeared a daunting task?
"On the page, I thought, oh God, this is impossible, but the grace of the play, I eventually found, is that it is possible, eminently possible.
"I didn't find it overly poetic", she adds, responding to a criticism that has been curiously levelled at the dramatist's work. "I felt that he gave short, sharp sentences when you needed them, wit or humour when you needed it, and then just when you want it, there is a lyric, tragic cadence."
Preparing the role, Cusack flew into a trademark tizzy of laborious background work.
"With Mai, I researched a massive amount. I researched with cancer surgeons and nurses, I went to Sligo and Galway, I found her university records, went to houses shed lived in, talked endlessly with people. It was a vast amount of work."
The presence of the author at rehearsals in London (the Out of Joint company first produced the play for the National Theatre, and brings it to the Gate from September 15th) must have kept the tension good and taut?
"Well, Sebastian was there every day - and some days that's the best possible set-up. If things aren't working out, you can go into that collaborative effort between writer, director and actor, which is wonderful. Then there are the other days. You look across the table and see his poor face looking at what you're doing with his work and it's so pained, he's absolutely miserable! Those days can be heart-breaking.
Max Stafford Clark, a notorious perfectionist in his methods, directed.
"He has a very particular way of working, very precise, very specific and I think without him I'd have found the play even more difficult. With every sentence in the play, he makes you find a transitive verb that you ally with an action, so each line has a different gesture. This means you won't just put up a broad brush stroke of anger, or whatever. It's very easy to end up doing that broad stroke of pain or anger or love, and you couldn't do that with this play, it would diminish it."
Cusask is in west Cork for a couple of weeks, pottering about in her castle with husband Jeremy Irons. ("Oh come on, it's not really a castle, it's more of a keep.") They have a son at Trinity, another who is 12. They divide their time between west Cork ("The clarity of the light!") and a spread in Oxfordshire, with some other necessary diversions thrown in. ("I absolutely have to get to Dublin three times a year for my hit, my Dublin drug.")
Theirs is apparently an almost unfashionably solid showbusiness marriage, and in Britain, still their most frequent home, Sinead and Jeremy are a prominent pair in the liberal establishment. This week, their names appeared on a list of celebrity contributors to the coffers of New Labour.
A far cry from the mid-1970s, when Sinead first settled in London, broke, eating beans, and in something of a gloom after being turfed out of the Abbey.
"I just wasn't very skilled at that time. I never went to drama school and I didn't have the craft and it showed. Maybe the Abbey were a bit . . . what's the word . . . well, maybe they should have given me a little more time before booting me out as they did. Ernest Blythe said I couldn't be heard past the first three rows. I suppose I took my time, but eventually I did work like a navvy at improving my craft."
The initial gloom soon shifted and she settled in.
"It was very exciting to be living in London, I was surrounded by other ethnic minorities, it was the 1970s . . ."
And you were knocking around with Georgie Best? "That class of thing, yes."
The Posh Spice of your day? "Sweet of you to say so."
Cyril, a famously strict Da, can't have been over the moon, his young wan and Bestie all over the papers? "Well the entire period is sort of a blur to me but I think it kind of amused him. It didn't amuse me, I thought it'd be the end of me, I'd never be taken seriously and it's extraordinary really that 25 years later, you're here asking me about my dalliance with a football player!"
Suitably chastened, the tabloid hound within sent howling from the hotel lobby, we move on to her period of emergence on both stage and screen. The navvy-like toil paid off, and she eventually arrived at the thespian Mecca of the RSC.
"I was passionate to play the classics and when I did, I was in a heaven I never thought I'd ascend to."
The classical repertoire completed, she now seems more inclined to pick and choose from the better new work. After Our Lady, there is a brace of films: an Australian effort, My Mother Frank, and Passionate Mind, set in France with Demi Moore in the lead.
"I play her shrink, actually . . ." (attempts to avoid cackles, fails.)
An old hand at both, she flicks back to film from theatre pretty smoothly.
"The base line is that it's all to do with truth, but your way of affecting that truth is different. In film, very little vocally or facially is required, body language must be minimised."
With the chislers nearly reared, she might be forgiven for taking things a little handier, hanging out in the castle (sorry, the keep) and that type of caper. But there's no great urge to slow down.
"I would hope not to. I want to feel myself committed to creative work and if it's not acting, then it may be something else, maybe writing. I have been approached by a publishing firm to maybe think in terms of fiction."
The Cusacks remain a tight-knit clan and Sinead gets together for endless natters with Sorcha and Niamh and the rest of the tribe.
"There's always a lot of chat, a vast amount, and yes we're all emotionally volatile so sometimes there's strain. But were very close."
As of the moment, there are no signs that her own offspring are planning to follow in their parents' actorly footsteps.
"If they are, they're keeping it pretty close to their chests. I want them to have as many options as possible, but I wouldn't discourage them. I would say `just because your father and I have had a modicum of success, it doesn't mean you will'. Rejection, serial rejection, is part and parcel of this whole thing."