At quarter to 10 on a Thursday morning, there are still 15 minutes before the Central Library opens. But a small group of people stand around its roped-off stairway in the middle of Dublin's Ilac shopping centre, peering up at the windows. They shift from foot to foot, check their phones, leaf through papers. One man stares at his watch. Then he holds it to his ear to make sure it is still ticking.

When a librarian appears to remove the guard rope, they stream up the stairs. Elsewhere in the centre, it's as if she has flicked a switch. A collection of people who might previously have appeared to be browsing through shops, or idling on benches, suddenly gravitate towards the entrance. These are only the first of around 2,000 Dubliners who will visit the library before it closes that evening.

In a recession, people remember the places where they can get something for nothing. The Central Library was built during an economic downturn, in 1986. Now it is seeing another one, the staff find that people are turning, or returning, to it. "All our indicators are up," says head librarian Bernadette Cogan. "Visitor numbers began to increase last year, and book issues are up about 10%."

Cogan, a slim, businesslike woman with a friendly face, is backed up by the lending librarian Aisling Murray. "Right across Dublin there's been an increase since the recession," she says cheerfully. "People have time on their hands, they're not going out as much. Or they don't want to spend 20 quid on a book in the bookshops."

As if to hammer this economical message home, the slogan "Join Your Local Library" appeared on billboards and bus stops around Dublin earlier this year, as the city's libraries mounted a major promotional campaign. In a way, it's surprising that libraries should require marketing. Doesn't everybody know what a library is for?

Not really, says Aisling. Libraries "have changed a lot," she explains. "And the changes happened literally over 10 years." The library, she says, is a mirror of society. "Every time something changes in the public domain, it changes in libraries as well. And it has to. We always have to make ourselves relevant." She suggests that people who last went when they were children may not know what the humble library has to offer.

Bernadette agrees. "If people haven't used a library in a long time," she says, "they may not know the extent of the services. While borrowing books is still the significant use, there are so many other uses. The internet in particular."

The internet is a mixed blessing for libraries. It has rendered a lot of their functions obsolete. But as online activity becomes ever more central to our society, those without it – or without the means to get it – are excluded. Public libraries can offer internet access to people who, for whatever reason, would rather not pay for it. The Central Library offers free internet, either through its own PCs or – if you bring in a laptop – on wireless. The branch's wireless area, Bernadette says, is full "from 10 o'clock in the morning. People are waiting outside."

The library building is every inch a municipal space. It bears all the scars of public service on a tight budget: grubby, scuffed carpets, cinderblock walls, a motley variety of shelving. Along practically every surface are pinned notices in jolly word-processor fonts – "Strictly No Eating Or Drinking"; "This Is Not A Seating Area" – and a facsimile of Uncle Sam: "I Want YOU – To Turn Off Your Cell Phone". But its slightly down-at-heel air is not unfriendly. And the wireless table is, indeed, packed with laptops.

At one end are computer-science students Abbas and Bruno, studying for a programming exam. "We come in maybe one day a week," says Bruno. "More before exams. It's great, it's a quiet place." They praise the staff, the resources. The only problem, in fact, is the capacity: there isn't enough room for everyone. "The worst thing is finding a plug for your laptop," Abbas says.

Abbas and Bruno are not Irish, and nor, it is clear from even a cursory look around the chairs and tables, are a fair number of the other people using the facilities. No less than 124 different nationalities are registered for the library's language classes. One young Brazilian man sits reading the Evening Herald while he waits for an English class to begin. He arrived in Ireland eight months ago. "I started coming here for the free internet," he says. "Then I saw the lessons."

"When this library started," says Bernadette of the classes, "it was all people learning foreign languages. But from about 2001 onwards, it's reversed completely." And the change is enormous. Now, about three-quarters of people are trying to learn English – though interestingly enough, the second-most popular language is Irish."

This is all part of the library's mission to be an inclusive place; not just somewhere you get something from a book, but somewhere you get something from a community. "A big aim for me," says Aisling, "is that libraries are set up to cater for everybody. And particularly the people on the margins of society. It can be a support service really for some people. Especially, say, for people who come into the country. They're trying to learn English, they're trying to meet other people of their own nationality. And get to know Irish people."

Bernadette, too, is keen on the idea of libraries as little societies in themselves. "They're community-based," she says. "There's this whole sort of social dimension. People come in on a regular basis, and you get to know the regulars." She introduces me to Hiroe, a young woman from Japan who attends the weekly mother-and-toddler groups in the children's library. Today Hiroe is here with her 15-month old daughter Fumino. Fumino is reading a Peter Rabbit book, or at least tugging happily at his head. A piano teacher back in Japan, Hiroe arrived in Dublin eight months ago with her doctor husband. On seeing Bernadette, she is all smiles, and it's easy to imagine what the library might mean for a new mother in a new country.

But it isn't just migrants, of course, who might need support in a community. "For some people," Bernadette says carefully, "it provides contact with a very safe service. And a much appreciated service, for them. The fact that people can come in and have many purposes, or no purpose – just to walk around..." She pauses. "There would be individuals for whom this library is a lifeline. There really is."

At one o'clock on a Wednesday, every chair is taken, every desk in use. An atmosphere of quiet studiousness prevails. Two pensioners sit in matching postures at the end of parallel shelf stacks, newspapers propped on laps. A girl walks by wearing an Islamic headscarf in a teenage-rebellion skull-and-crossbones pattern. And it's not difficult to spot individuals who bear the obvious hallmarks of vulnerability – a tight hold on a plastic bag of miscellaneous possessions, a moving lip or shaky hand.

If anything, however, the impression is not of a group that society might be at risk of leaving behind, but of people in progress – people who are, both metaphorically and literally, going places. One man studies diligently with a wheeled suitcase at his side. Brightly coloured trainers and school uniforms are numerous; the boy at the next table is reading a book called Leaving Cert Art History – Less Stress, More Success.

And this momentum is not restricted to the very young. Anna is 38 and from Tanzania; though as she arrived nine years ago, Ireland no longer holds many surprises. ("I hope I never get used to the weather," she remarks wearily.) She comes to the library to study for her accountancy exams, after dropping her daughters off at school. "I used to just come here and borrow books and videos," she says. "But it's good for studying. People mind their own business."

She's right – the library might be a community, but it's also a place where people can be private. To glance down a line of desks is to look into a neatly aligned row of different worlds, connected only by the tables they happen to be sharing. There's room, it seems, for everyone. Except when there isn't. "It's packed now," says one white-haired old man reading the Examiner. "Not like a couple of years ago. Especially in the afternoons, it's hard to get a seat now." What does he do in the library? "I get the paper. Sometimes I use the internet."

He doesn't look like your average web surfer. But teaching older people about computers is part of librarian Barry Meggs' job. "The courses are open to practically anybody," he says, "but they're invariably pensioners who come to us. People who didn't work with computers before they retired. The story I've heard, oh, I don't know how many times, is, 'My kids have grown up and they're living in Australia. And they got me a laptop so I could keep in touch. How do I keep in touch?' So we literally start off with, 'This is a computer. This is the 'on' button.' They think it's brilliant. It's a whole new world."

One of the pupils in the so-called 'Silver Surfer' class is Pauline, from Ringsend, who is hoping to keep some new-found computer skills up to scratch. "I want to look things up on the internet," she says. "All the great deals on holidays." The classes may be new, but Pauline is a lifelong library fan. "I've always had a library card," she says, seriously. "I mightn't have had much else but I've always had a library card."

This afternoon's course is being run by Catherine O'Callaghan. ("You should get that lady's name," Pauline tells me. "She's very nice.") When the four students are set up and sitting in front of Google, Catherine asks "What do you want to do?" And the pensioners, too, are going places. "I want to go onto Aer Arann," says one. "I want to look at Aer Lingus," says another. They all want to book flights.

Catherine shepherds them each to the right airline. "Now," she says encouragingly, "from the drop-down menu, pick a city you want to go to." Discussing possible dates and warning of hidden handling charges, she is more amicable travel adviser than librarian. "I want to go to Seattle," says Pauline, calculating prices.

The Dublin libraries also run book clubs. The clubs are based partly on the interests of the staff members leading the group. Barry Meggs' niche is science fiction. "It was Aisling's idea to do something genre-focused," he says, dryly, "and since it was falling to me, I wasn't going to do romantic fiction or Regency classics." In the last meetup, he says, they fell to talking about gender roles in Ursula Le Guin.

Like everyone else I speak to in the library, from Jimmy the security man to Peter the former ice-cream distributor who runs the business databases, Barry is voluble and enthusiastic. The pride he takes in his job is plain to see. So what's the satisfaction in being a librarian? "There's a sense of being appreciated," says Bernadette. "Sure there's challenges, there always is. But when you support somebody to find something, or to use the library for their learning, or just to enjoy it with their children… You just kind of say, where else would people have that opportunity?"

Aisling has a slightly different take. "For me it's the social aspects," she says. "Maybe not so much here, because it's bigger, but especially in the local libraries you get to know all your regular borrowers. It's very social." There is a pause. "And that old-fashioned thing as well – I always loved books. So to actually work with something that you love, and have a passion for… It's great." She smiles. At eight o'clock that evening, the library's staff and customers will go their separate ways. But some of them, at least, will be back at quarter to 10 tomorrow.