Six novels in, and writing isn't getting any easier for Alice McDermott, but sitting down at her desk each day is the best part, she tells Arminta Wallace.

There are nodding moments on almost every page of Alice McDermott's new novel, After This. You don't know what nodding moments are? Of course you do. There you are, reading happily along, when something in a narrative makes you pause. Maybe you're startled, maybe moved. Maybe amused. Then you nod briefly to yourself - physically or mentally, it doesn't matter - and read on.

After This is a tapestry woven from such moments of recognition. The story of the life of a family in New York, it's framed by two devastating wars - the second World War and the Vietnam War - and its set-pieces unfold so beautifully that when you're not nodding, you will, as likely as not, be doing that sharp-intake-of-breath thing.

Few writers recreate the tragedy of the commonplace with greater deftness than McDermott. Her fourth novel, Charming Billy, beat Tom Wolfe's A Man in Full to the National Book Award in the US in 1998 - though her fans will tell you that her previous outing, At Weddings and Wakes, is a better book. In 2002 she published the much-lauded Child Of My Heart, and After This crosses the Atlantic garlanded with rave reviews.

Part of the reason McDermott's world is so recognisable to us is that it's the Irish-American world of the second half of the 20th century. Is it, though, a world that's on the wane? "As a matter of fact I think it may be increasing," McDermott says. "In America, fifth- and sixth-generation Irish are claiming their roots more and more. There's a huge interest in Irish music, language, culture and history. Irish-studies courses are blossoming in universities all over the place. And I don't think that's a bad thing - though it probably says more about what's going on in the United States than anything else. People are feeling more and more detached from America as a homeland, and more and more detached from individual communities."

In person, McDermott is tiny and calm and impressively articulate. Second-generation Irish herself, she grew up in the kind of community she writes about with such understated empathy. "I was raised with the idea that, 'Yes, you're Irish and of course that's the best thing to be. But it's also not polite to throw it in anybody's face. Because, hey - the Italians will feel bad if you make a big deal about it.' Part of it was false modesty. But the other part was that, growing up in New York, everybody had grandparents who were from some place else." Because of her subject matter, McDermott is often labelled "an Irish-American writer", or "a Catholic writer". How does she feel about these labels? She spreads her hands. Like most writers, she's torn between wanting to be on the shelves and not wanting to be restricted to certain shelves only. "For me, the 'Catholic' is probably more important to my work than the 'Irish' part," she says. "I see that in a lot of ways - primarily in the way I use language. I'm influenced by the rituals of prayer and the language of the church. That's where I first heard poetry, for instance.

"Through their Catholicism, my characters are provided with a language for things they wouldn't otherwise have a language for. The sense of longing; the sense of yearning; the sense of hope across the highs and lows of daily life; all these are things those characters would not be discussing in philosophical terms. That's why Catholicism is so tied up in my writing. Am I happy about that? Would I like it to be otherwise? Often, yes, I would like it to be otherwise." But, she says with a rueful smile, there's no getting away from it. Sure, she has been tempted to make a dash for a different set of tracks - write something set in a totally different cultural milieu. It is, she grins, a temptation she has managed, so far at least, to resist. "It would be writing a story from the wrong end," she says. "To prove that I can. That's when you lose whatever integrity your characters may ever develop."

With three sons aged between 14 and 25 and a neuroscientist husband, McDermott is as aware as anybody else of the hostility the very word "Catholic" often incurs in the secular world. However, she insists that the Catholicism of her characters is quite separate from the politics of the Catholic Church. "I don't ever set out to define Catholicism, or redeem Catholicism, or make excuses," she says. "It's an aspect of being a so-called Catholic writer that I can't allow to influence the work itself. If that were to seep into the fiction, it would become polemical. I write to find out what's at the heart of all our experiences, and I do that through individual characters who often happen to be Catholic. Not necessarily good Catholics, mind you."

McDermott says people often ask how, having seen what the church has done in the world, she can stick with it. "My answer is, 'Well, when I see what the current administration in the United States is doing, how am I keeping my passport?' If we left every nation that made terrible mistakes, I don't know where we would live. The spiritual life itself can still have great value - despite the flaws and failings of the institution that has delivered that life to us."

Much the same might be said of families. They can be mad and dysfunctional and places where terrible mistakes are made - but they can also be places of comfort and support. "My first impulse for this book was to give some sense of family as our first shelter," says McDermott, "the place where we hunker down. Okay, we all know that family is not the great thing that we used to believe it was - but while one's first impulse might be to write about what makes a family dysfunctional, I wanted to write about what keeps a family going. The wonderful irony is that in order for a family to be successful, it has to break apart so that other families can form. That dynamic, that movement, was interesting to me."

The action proceeds through a series of pivotal moments in the Keane family's life. The meeting of John and Mary; the birth, in their living-room, of their third child; the destruction wrought by a hurricane in their garden; an outing to the beach. In one particularly striking sequence, Mary and her daughter Annie queue for hours to see Michelangelo's Pietà at the New York World's Fair. Each of the snapshots is as self-contained as a short story: each is a chronicle of ordinariness. The cumulative effect, though, is extraordinarily subtle. Terrible world events blow through these pages: like the wind, they're not seen directly, but only in their consequences.

Light relief, of a kind, is provided by Mary's unmarried friend Pauline, whose embittered voice runs as a kind of dissonant counterpoint through the book. Now here's a type - the miserable spinster typist who's invited to Christmas dinner - which is definitely an endangered species? "I can pick out Paulines from every generation," says McDermott firmly. "They're different now, but they're still there. That neediness; that impulse to generosity that's immediately pulled back. Paulines are a force of nature. I've had letters from people telling me the names of their Paulines. Of course, there are also the Marys. Paulines pick them out - the people who'll be good to them, who won't tell them to get out and shut the door. I see them in the younger generation, too."

To McDermott's amusement, the book's title has given rise to considerable debate on the part of reviewers. The phrase "after this our exile" is from the Salve Regina, which is recited, in a pretty unusual context, by one of the characters. "Often," says McDermott with a chuckle, "I think I'm being so blatant, so obvious that I'm almost embarrassing myself. When I called my editor - a brilliant guy with whom I've worked from day one of my publishing career - and told him the title, he said; 'Oh yes. Yes. After this, joy in the morning. Joy after sorrow. And one decade after the other. It begins after the second World War, ends after Vietnam. That's wonderful.' And I said, 'Yes - and the prayer. And after this our exile. We're in exile. Paradise lost, you know?' Silence at the other end of the line. And then he said, 'Oh, nobody's gonna get that'."

When McDermott was a young college student toying with such trendy subjects as sociology, an astute English professor told her she was a writer. "If he hadn't," she says, "I don't know how long it might have taken me to work it out. Maybe I never would have gotten to it. It was a life-changing moment, and part of the reason I continue to teach is because of that moment in my own life." She is currently writer-in-residence at Johns Hopkins University in Baltimore. "The nice thing about working with young writers," she says, "is that they're struggling with the same things I'm struggling with." Writing doesn't get easier, then - after half a dozen novels? "It doesn't get easier. As I tell my students, every time you begin a new story, or a new novel, or an essay, you are a novice. This is a story you've never told before. You've never had to choose the word for this sentence, never created this character or this situation. There can be similarities from one book to another, but you gain no points for having done it once. You have to teach yourself every time.

"But the more I do it, the more I understand - and maybe this is the thing you know, six novels later, that you didn't know with the first novel - that the hardest part is the best part. Getting to your desk every day and seeing what happens. Even when you're throwing out what you wrote the other day, or two weeks ago. That's the best part. It's lovely to have good reviews, do a book tour, see your book in print. But the pleasure and the reward are all in working with language - using this beautiful tool to say something that's authentic, to remember what makes us human. And it's a bitch. I'm a miserable person when I'm doing it.

"Why do I do it? I go to fiction for the moments that are beyond language. It's the same thing you go to art for, or music for. A moment of transcendence. I see something. I can't quite grasp it, but it's something that gives me joy, gives me hope. Makes me think we're not as bad as all that."

After This by Alice McDermott is published by Bloomsbury, £10.99