Never mind speculation over his single status, the real question most fans want to ask about George Clooney is: does he really drink Nespresso? With homes in both Burbank, California and Lake Como in Italy, chances are that the actor could easily hire his own private barista to freshly grind beans for his morning espresso, as opposed to just slotting a coffee 'pod' into the machine as he claims in the celebrated advertisements. One thing is certain – Clooney imbues the brand with his own languid glamour and cool good looks. Closer to home, Diarmuid Gavin, a man more associated with exclamation-mark garden makeovers, is set to emulate the Clooney effect in commercials for instant-coffee brand Kenco. Attaching a household name to a household product is the advertising world's most successful marketing approach, while the celebrity endorsers, or 'brand ambassadors' as they are loftily known in the business, earn a lot more than just a year's supply of coffee beans. Sainsbury's paid chef Jamie Oliver £1.2m (€1.35m) a year to promote its products – or at least it did until his wife Jools was photographed, three bags full, outside rival supermarket Waitrose. What sort of money is involved for Irish celebrities? Ball-park figures can be anything from €500,000 to €2m for a contract with one of the big multinationals. The duration of such deals is usually three to four years and most likely covers more than just a single commercial – the celebrity in question may be required to attend promotions, or act as host at the product launch.

But just how persuasive are celebrity tie-ins here, where companies are restricted to a smaller pool of famous faces, and tighter budgets? Paul McCabe, managing director of MCM Communications, a division of the McConnells Advertising Group, says success using Irish names is down to the age of the market being targeted.

"Certainly if you are marketing a product towards 16-year olds, it's unlikely someone from Fair City is going to make an impact, as opposed to an international star," he says. "It would be like pitching a product endorsed by a player from Shelbourne, rather than a big name from Manchester United, for example. The references of younger consumers are more global than local."

Newbridge Silverware would appear to have taken that approach with its more recent choice of celebrities. Where once the home-grown glamour of Rosanna Davison and Samantha Mumba sufficed to deliver the company's jewellery ranges, it is models such as Yasmin le Bon, and now Sophie Dahl, who bestow an international cachet to the brand. And yet, there is one hugely successful band of Irishmen currently carrying a level of international clout that companies can only dream about – Ireland's Grand Slam-winning rugby players.

"Obviously, hiring big names can be a gamble for companies, but sports stars are undoubtedly one of the safer bets," says Michael Cullen of Marketing magazine. "They tend to lead healthier lifestyles that won't impact negatively on a product. Ronan O'Gara is a good example of someone that is well liked, not just by Munster fans, even though he tends to keep his hands in his pockets at VIP events."

The phenomenal rise of Irish rugby has proven fruitful for a long list of companies. It's hard to envisage Lions captain Paul O'Connell without a bottle of Powerade clenched in his mighty fist. His fellow Lion Brian O'Driscoll is something of a marketing groupie. Not only is he "the official rugby ambassador for Gillette" – assuring one doubting interviewer, "Well, obviously I use the whole Gillette package, everything from the new razor to the shaving balm" – he's also on the Powerade roster, although the Leinster man almost overdid the brand-loyalty thing when, several years back, he was filmed clutching a bottle prominently throughout a post-match interview, and was subsequently lampooned by George Hook.

O'Driscoll's former girlfriend Glenda Gilson promotes the efficacy of a product somewhat more difficult for consumers to swallow – breast-enlargement pills. The Irish company behind the pills, Lifes2good, uses celebrity endorsement as its main marketing tool for a range of health and beauty products. Christy O'Connor junior has enthused about the benefits of magnetic therapy, while Amanda Brunker has tapped into the national obsession with weight loss by endorsing slimming tablets she claims helped her drop four dress sizes. Standing over all of these claims is Lifes2good managing director James Murphy, a celebrity himself – he's one of the Galway Tenors trio. Murphy looks for credibility, as well as celebrity, when signing well-known names to endorse products that invariably involve a big leap of faith on the part of consumers.

"When I go looking for a celebrity, I need to find someone who is genuine," he has said. "I don't want someone who can't stand over the product – they have to be able to say that they used the product. And that it worked."

But consumers who are influenced by celebrity endorsement are not just buying the product – they hope to acquire some of the qualities of that famous name too.

"Celebrities have real 'stopping power'. In a fragmented, cluttered world, famous people stand out," says Rosie Hand, programme director for the MSc in advertising at DIT Aungier Street. "The positive aspect of the celebrity relates to the brand, and those who buy feel they take on some of the celebrity's characteristics. But one of the core tenets of this type of marketing relates to the long-term relationship between both the brand and the endorser. Companies who base their entire sales strategy on endorsement run risks with the celebrity whose lifestyle takes a turn not complimentary to the product."

Kate Moss was famously dropped by Chanel and H&M after being exposed in a newspaper photograph taking cocaine. But ultimately it proved a mere blip for a woman who is practically an advertising billboard, with other companies subsequently lining up to hire her to promote their brands. "In a funny way – and whether we like it or not – Kate Moss being snapped snorting coke may actually have enhanced her cool image," says Paul McCabe.

Footballer and actor Vinny Jones's bad-guy image was a huge factor in his being hired, bewilderingly (and briefly), as the face of Bacardi Rum. The drinks company wanted to reposition rum and coke – traditionally seen as a woman's tipple – as a drink appealing to men. But when Jones was found guilty of air rage and assault of a passenger, the company decided his version of macho glamour wasn't what it was after.

Another problem with celebrity endorsement is that it is increasingly felt people are becoming confused by a seemingly never-ending gallery of famous faces promoting products. "The old battle lines between Coke and Pepsi, Madonna v Britney, don't engage people anymore. And endorsement fatigue is so obvious now – the Spice Girls were like a franchise, and similarly Girls Aloud look like they'll promote anything," says McCabe.

But endorsement by a big name can sometimes be more subtle than just plonking a cardboard cut-out of some celebrity beside a product. Who would have thought, for example, that the plummy-voiced woman behind the televised Marks & Spencer food adverts is none other than Irish actress Dervla Kirwan – best known for pulling pints in Ballykissangel?

Cars are given as 'gifts' in the hope that the celebrity in question will be photographed driving that particular make. The so-called 'red carpet' shots are often thinly veiled ads for designers ? the Oscars being the biggest offender here. Or the celebrity actually becomes the 'designer', such as Moss for Topshop, or Lily Allen for New Look. "Once it became apparent that celebrities could sell clothes, asking them to put their name to ranges cast in their own style wasn't a great leap. And it's celebrity style that's for sale, not celebrities' clothes," Vogue editor Alexandra Shulman has said.

But with consumer confidence at an all-time low, is it integrity, rather than celebrity, that companies concentrate on now?

"There are signs of a return to retro marketing, such as in the new Persil ads," says Rosie Hand. "I feel that celebrities who encapsulate what could be called old-fashioned, traditional values will have more currency. By that, I certainly don't mean people who are dowdy, but those with a practical, genuine image easier for people to buy in to." Chef Neven Maguire already had a relationship with classic Belleek pottery before they hired him as brand ambassador for a tableware range this year. "The company is literally down the road from our restaurant, and it seemed natural for me to promote the brand as there is an authenticity in that collaboration," he says.

In certain instances, both the brand and the celebrity are so equally iconic that it's more about mutual admiration than money. U2's passion for the Apple iPod – "the most interesting art object since the electric guitar in terms of music", said Bono – famously led to the band's deal with Apple to advertise their 2004 album How to Dismantle an Atomic Bomb with a special-edition iPod. The deal was said to be worth €30m – but it is believed no money changed hands.

Association with worthy causes, where commercial gain is not a driving factor, can give the famous a degree of integrity that enhances their public profile, and also challenges perceptions. Unicef Ireland has a host of ambassadors, from Ryan Tubridy to Colin Farrell. In the UK, Keira Knightley appears in a public-service ad on domestic violence.

As for Clooney, seen as one of Hollywood's more politically active, and politically correct stars, there is no conflict in ideals over his promotion of Nespresso. He found it "irritating" when asked about the brand's parent company, Nestlé, which has been widely criticised over its promotion of powdered baby milk in the third world. "I'm not going to apologise for trying to make a living," Clooney has said on his endorsement. Stars who choose to sell their celebrity status, if not their soul, to endorse commercial products, are under scrutiny in these more ethically enlightened times. The millions spent by advertising agencies on persuading the famous to lend their names to brands is increasingly seen by a sceptical public as, at the very least, unnecessary.

After all, there are only so many grease-encrusted George Foreman steak fryers the average kitchen press can contain...