As a young girl in Cork in the 1960s, actress Fiona Shaw was forbidden only one thing by her mother: to become a nun. The Ireland of her childhood was effectively a theocracy; the Roman Catholic Church exerted a mesmeric hold over the country. Shaw grew up with a solemn awareness that without a “sense of sin and guilt” there is no genuine religion.

She left Cork at the age of 21 to train at RADA, and all through her years of success at the RSC and the National Theatre she lived a resolutely secular life - the life, she says, of a “perfect bohemian”. If she was not being acclaimed as the next Maggie Smith or Vanessa Redgrave, she was moving with the smart set in Los Angeles, where she is rightly admired as a comedienne and quirky character actress. After she played a harassed English schoolmistress who was farcically infatuated with Tom Selleck in Three Men and a Little Lady, the big offers rolled in from Hollywood, including one for her own weekly sitcom.

Yet something was missing from her life, something she now describes as an absence of spiritual ritual. In interviews she sometimes spoke of her fascination with nuns and of how her talk, full of theological speculation, constantly bumped into Catholicism. “If we had parallel lives, there would be a bit of me that would be a nun,” she once said. “I admire contemplative lives. It’s a constructive way of opting out.”

In January this year Shaw, 38, finally opted out constructively when she spent two weeks with the Tyburns, an order of Benedictine nuns, at their convent on the Bayswater Road in London. The only rules laid down by the Mother General were that she had to wear something modest (she chose a long, brown woollen dress) and submit herself entirely to the discipline of the convent.

The nuns were of mixed nationality - English, Scottish, Australian, New Zealanders and Peruvian. The oldest was 96; the youngest in her 20s. Founded at the end of the last century, the Tyburn nuns - “adorers of the Sacred Heart” - are an enclosed, contemplative order whose lives are devoted to the perpetual adoration of Christ.

Waking at 5am each day, they are fiercely ascetic. It is, says Shaw in her spoken diary of her stay in the convent, as if the nuns are pursuing a kind of poetic suicide.

“In a real sense,” she says, “the nuns are dead already. Or rather, they are in a heightened sense of preparing to die. Most of them have deliberately lost touch with their families. They have let them go. They are simply clearing a pathway to God. The idea is that if there is a God and you empty your mind of the clutter of existence, then you might just catch Him.” What initially struck Shaw about the nuns was the monotony and repetitiveness of their routine. Each day is the same. There is no respite from the fervour of their adulation.

Their routine is structured around an austere ritual of prayer, song and worship. They rarely interact with each other or with the outside world. Recreation is limited to one hour of occasional indoor games - pool, Scrabble, snakes and ladders (Trivial Pursuit is deemed “too world ly”) - and walks in the garden. There is a small library of general books but the Bible remains everyone’s preferred reading. They have no television, CDs or records; the radio is permitted only when there is a major incident. “Like when there is a bomb?” Shaw asked the nuns. “Oh, no,” replied the Mother General. “Like when the Pope dies.”

The last time the nuns listened to the radio was in July 1981 - the day of the royal wedding.

Meals are regular but basic - there are no luxuries. “Chicken and pasta is about as good as it gets,” says Shaw. Each night dinner is interrupted by a bell, which prompts a moment of intense, silent contemplation. Indeed, the convent resounds to the sound of silence: the nuns are quiet for 22 hours a day.

On the third evening of her stay in the convent, Shaw “hit bad weather”. Watching the nuns at dinner she was struck by their apparent gloominess. “It isn’t natural,” she whispered into her tape recorder after retiring to her room. “I look at the nuns and I don’t see any great vision on their faces. They are bored, too. They are bored and they have a lifetime of this to face.”

Shaw’s diary raises provocative questions. The nuns, some of whom lived conventionally before joining the convent, are adamant that they are the brides of Christ. “But to be in love with Christ is to be open to Him physically,” she says. “One of the main experiences of falling in love is to have a heightened sexual awareness of the other. My experience of the convent was the absolute absence of the language of sex and sexuality.”

Later, she asks: “What happens to the sexual energy of the nuns, especially as they never exercise? Why do you never see any flesh or hair? I don’t know what happens if you have a sexual problem here; if you become obsessed with a workman or something.”

She speculates, too, on what drove them to enter holy orders, wondering if they are in some way damaged or depressed.

There are also some jewels of self-revelation, as Shaw fantasises about the nuns engaging in secret orgies with monks, confesses her inability to concentrate during prayer, regrets that she cannot leave with the workman who wanders around the convent carrying an enormous plank of wood behind which he hides, and dreams of diving into a pool of red wine.

But midway through her stay Shaw’s perception of the nuns began to turn. As they prayed, their faces “radiant with the joy of their love of God”, she felt that they were free in a way that she was not. “I swung from loathing their lifestyle to admiring it beyond measure. I am completely taken with their way of life. That power to yield yourself to something while at the same time retaining complete control of yourself is magical.”

After leaving the convent at the beginning of February, Shaw, who lives alone in a small flat in Primrose Hill, London, found that the experience stayed with her in unexpected ways. After an initial celebration, she began waking at 5.30am - describing it as if she could feel the prayers of the nuns on her. As well as giving up alcohol and being “dejected by the excesses” of consumer society, she is even questioning her profession.

“I now feel very ambivalent about what I do. I’m bemused at how much acting I’ve done. I’ve been doing it non-stop since I was 24. You become a sort of acting machine; it makes you wonder if you are wasting some of your best energy.

“I did The Taming of the Shrew for nearly two years and I can remember only four or five nights of that. I don’t want to sound Pollyanna-ish about this, but we have become such a cynical, prurient generation, so wrapped up in ourselves, that we have lost sight of what is important and true.”

She is moved by the great biblical paradoxes: that we are at our richest when we have nothing; that it is only in darkness that we finally learn to see. “A couple of years ago these nuns were burgled but the burglars took nothing - because they had nothing to take,” she says. “This joy of having nothing means, of course, that you have everything.” On the morning of her departure, Shaw told the Mother General that if she had two lives, one would be spent inside the convent. The old nun smiled benignly, reached out to touch the young actress and said: “Yes, Fiona, but you have only the one life.” Shaw says: “I knew at that moment that it was right for me to rejoin the world but that I would do so a changed person.”