It's often hard to be surprised by a normal shop's stock. You might have a favourite outlet, but you'll usually know more or less what you're going to find in there, particularly if it is a chain, no matter how upmarket it may be. This is what is so marvellous about charity shops: each one is a unique box of surprises and treasure to be explored by those in search of bargains, the esoteric, and the unexpected: a sort of Lucky Bag for adults.

I have long been a huge fan of charity shops, and they have always rewarded my visits. At least half my wardrobe comes from them, as does all my kitchen crockery, several pieces of furniture and innumerable books. Some of my all-time favourite finds over the years are: a perfect art deco dressing table for £60; an oak hall table for £25; a 1950s black velvet, satin-quilted opera coat, a 1960s Mary Quant suede dress and a Victorian astrakhan jacket, all for a tenner each; a sterling silver vase, a still-boxed Waterford Glass goblet, an exquisite tiny etching of the Charles Bridge in Prague, a 1930s silk crepe evening bag, complete with kid-lined purse and a bevelled mirror within, and a set of charming 1950s kitchen curtains, all for a fiver each.

Two of the most prominent chains are Oxfam and Enable Ireland, which have 18 shops apiece in the State. Oxfam was founded in Oxford in the 1940s to aid famine relief in Greece after the second World War, and takes its name from the Oxford Committee for Famine Relief. It has had shops here for some 30 years. All its profits go to aid development in Third World countries.

Enable Ireland has been here since 1987, and its profits go to aid people with physical disabilities through Ireland. They have gone through a few name changes in recent politically correct times. Less than a decade ago, the chain was known as The Spastics Society. It then changed to Cerebral Palsy Ireland and shortly after to its current name.

There are many other one-off charity shops in towns throughout the country, as well as some that have just a few local outlets. Mrs Greene's, which opened in 1993, falls into this category, with three shops in Dublin. Its profits go to Dublin's Cheeverstown House, which promotes the integration of people with mental handicap into the community.

There is still a slightly ambivalent attitude towards buying from charity shops in this country, probably because the profile of people who shop there is unlike shoppers elsewhere. They are a combination of those who shop there because they like it, students on a tight budget and those who often cannot afford to shop elsewhere. Some of this ambivalence is generational, where people of a certain age and background would never dream of buying second-hand clothes, and some is straightforward snobbery which commutes across all generations.

At both Enable Ireland and Mrs Greene's, clothes, furniture, bric-a-brac and books form the stock, clothing taking up the most floor space. Only a small percentage of the stock donated makes it onto the floor: dirty, torn or resolutely unfashionable clothes don't cut it, although nothing is wasted since those items are sold on for recycling.

"We only put quality items on the floor, because we try to make the shops look like boutiques," Eileen Timmons, events manager of Enable Ireland explains. "A lot of people shop here because they have to, and we try to make them feel they are in boutique surroundings, with changing rooms and fashionable clothes, just like a high-street shop."

Oxfam shops are a bit different from other charity shops as, alongside second-hand items, they also sell a large number of new products sourced from developing countries: the Fair Trade range. Margaret Carr is their commercial product manager, and she regularly goes to the London office to choose items for the Irish shops.

"The basic criteria of choice is that the items have to be a fair price," she explains. This means fair to the person who made or grew them, and fair to the wider community from where that person came. A percentage of the money paid to craftspeople by Oxfam goes towards future development of the community. Fair Trade sells ethnic bags, clothes, jewellery and some foodstuffs, with coffee, tea, chocolate and honey their biggest sellers.

There are a number of methods used to collect donations: through clothes and books banks in car parks, house-to-house collections from pre-sent donor bags, and from people who deliver in person to the shop. Furniture can be collected by arrangement.

Prices tend to be the same in all the shops, usually under £5 for separates, and under £10 for dresses and jackets. Books are 50p to £2, and I've rarely seen any furniture beyond £80.

Most shops now have a rail with retro or antique clothing (usually for women), and these items are a little more expensive, but still way below what you'd find in shops dedicated to selling this sort of thing commercially. Designer labels also make their way in the doors, for a fraction of their original price.

Undoubtedly, the success of the shops lies in the ongoing dedication of their volunteers. Most charity shops have a paid manager, with at least one other paid member of staff, but almost everyone else volunteers their time for free. Oxfam, Enable Ireland and Mrs Greene's all agree that they couldn't run without their volunteers: Enable Ireland alone has some 300 volunteers for its 18 shops.

"They would mostly be older ladies," says Annette Oliver of Mrs Greene's. "But sometimes we get young people between jobs, and people seeking refugee status, who can't work for money."

Timmons agrees that the core of volunteers is, and always has been, older women. "But we're trying to interest young people into volunteering also; transition year students in particular."

While spokespeople for Oxfam, Enable Ireland and Mrs Greene's say the stigma attached to charity shops has disappeared, I have often noticed people looking distinctly uncomfortable when they've admired something I'm wearing, and are then told it's from a charity shop. It's probably too optimistic to think that charity shops will ever attain the social footing of their high-street counterparts, but they certainly attract a large and loyal customer base.

And for those who buy in charity shops, there is the satisfaction of knowing the money is going to help people who really need it, rather than towards the coffers of a purely commercial operation.