The great thing about writing a sequel is that you can go straight in with the action, and no need to worry about setting the scene. Colm Tóibín certainly does that in Long Island, the follow-up to his 2009 novel Brooklyn. That book shifted Tóibín from being a respected, prize-friendly literary novelist to a commercial success: his publishers’ publicity materials at the time accurately predicted that it would be his “breakout novel” , which would “do for him what Atonement did for [Ian] McEwan”.

Brooklyn succeeded artistically and commercially because it told a simple story well: a satisfyingly sad tale of thwarted love in 1950s Ireland. It featured Eilis Lacey, a young woman living in Tóibín’s old stamping ground of Enniscorthy, a town in County Wexford near the country’s south-east coast. In Brooklyn, Eilis went to the US and secretly married, came back to Ireland for a family death and then hampered her mother’s hopes by returning to America rather than settling down with local boy Jim Farrell. Her decision seemed to surprise Eilis as much as it did the reader: her dominant characteristic in Brooklyn was a maddening passivity toward her own destiny – at least, right until the moment she decided to return to America.

In Long Island, 20 years have passed and we’re in the mid-1970s: the date is never specified, but references to Vietnam and Watergate help orient us. Eilis Lacey is now Eilis Fiorello, living with her Italian-American husband, Tony, and teenage children, Rosella and Larry – ominously, Tony’s brothers and parents live right next door.

The drama Tóibín uses to kick off the story is a strange man calling at Eilis’s door; he tells her, in short order, that Tony, a plumber, has been spending a lot of time with the man’s wife and “his plumbing is so good that she is to have a baby in August”. Furthermore, “as soon as this little bastard is born, I am transporting it here. And if you are not at home […] I’ll leave it right here on your doorstep.” And so the moment of crisis, which in Brooklyn took a long time to appear, this time informs the whole book, and invests it with an appetising tension from the start.

Eilis is unsurprisingly thrown by the news, and doesn’t know who to turn to. “There must be some number she could call.” But we quickly learn that she has changed since Brooklyn: two decades of marriage, of creating her own family and dealing with Tony’s, have seen to that, and she is no longer meek or passive. When she argues with Tony’s father, brother Enzo asks Tony, “Can you not control her?” He can’t. “A part of her life was ended,” Eilis concludes, and now she must decide where the rest of it lies.

The answer, at least for a time, is back in Enniscorthy, where she heads for a holiday to mark her mother’s 80th birthday, bringing her children with her. (“You never told me about cheese and onion crisps or salt and vinegar,” enthuses Larry, discovering Ireland’s pubs.) This is fertile territory for Tóibín, who has always been good on mothers, from early novels The Story of the Night and The Blackwater Lightship to the story collection Mothers and Sons. Eilis, escaping Tony’s mother who has plans for the new baby, runs straight into the stubbornness of her own mother, who refuses to let workmen install the new kitchen appliances Eilis has bought her, and furthermore tells Eilis that she has sold her house and that Eilis won’t be getting a whisker of the proceeds.

Still, the gang’s all here: not just family but Eilis’s old best friend Nancy, now running a chip shop, and old flame Jim Farrell, who owns the family pub. Nancy and Jim have drifted together in recent years, sneaking into each other’s houses to sleep together, and have become engaged, albeit without much enthusiasm. Tóibín takes us into both their heads, switching between Eilis, Nancy and Jim, each voice as convincing as the others.

But it’s not only Eilis who has changed in Long Island: Ireland is changing, too. And change hurts: some locals are appalled by the smells from Nancy’s newfangled chip shop, others scandalised by boisterous singing at a wedding. Some things, though, stay the same: it’s clear that Jim never got over Eilis.

Long Island often reads like a masterclass in everything Tóibín can do. Minor characters are as well drawn as the main players: an irascible woman selling a plot of land (“she likes a fuss”, the estate agent warns the purchaser) only gets two pages but is a vivid highlight. There is subtle comedy – a man at a wedding directs Nancy around the floor “like a man driving a tractor” – and an eye for detail, as Nancy gets excited about the prospect of introducing the toasted cheese sandwich to Ireland’s pubs.

There are a few misses. Some characters – Tony’s brother, Frank, Eilis’s brother, Jack – seem to be there only to smooth the plot with unexpected interventions, but the plot doesn’t need to be smoothed. This is a book, after all, that shows how blind we are to our own motivations, and how resistant to rationality. Late in the story, Eilis tosses up reasons to do or not do a particular act, “but the reasons she thought of made no difference”. Long Island shows also our need to control those closest to us, and in this respect reminded me of Maeve Brennan’s great collection of Dublin stories, The Springs of Affection.

The plot picks up pace – perhaps too much pace – in the last 50 pages, where events pile up in a way that threatens to violate the story’s slow build, and characters behave with unusual cunning and elan. But with so many motivations muffled, who can say what is plausible anyway? Much of the interest in Long Island lies in what is not said: when someone is asked a question but doesn’t answer; when Jim finds that “there was nothing to say […] nothing he could find the words for now”; or when a character enters a room looking for someone, finds it empty, and the reader’s heart drops three floors. These silences and absences at the core of this subtle, intelligent and moving book mean the reader has to do a certain amount of work – but it is work very well rewarded.

• Long Island by Colm Tóibín is published by Pan Macmillan (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply