Mr Martin Glenn is one of the few Englishmen who will have something to celebrate on St Patrick's Day.

The national holiday will mark the fifth anniversary of Walker's Crisps' launch on the Irish market. The brand, once unknown outside the English Midlands, has come from a standing start in 2000 to take 20 per cent of the Irish market, and is now a serious challenger to the home-grown and once-dominant rival, the C&C-owned Tayto.

Mr Glenn is president of PepsiCo UK and Ireland, a $2.2 billion (€1.7 billion) turnover business that produces and sells brands such as Walker's, Doritos, Tropicana orange juice and 7Up, and owns the Pepsi soft drink franchise. It buys supplies from PepsiCo Manufacturing in Cork but he does not have responsibility for that operation.

Despite its name, PepsiCo in this part of the world is more snack foods than soft drinks. Walker's is one of its flagships. Like Tayto, which began as a one-man business in Dublin, the brand had humble beginnings - a pie shop in Leicester to be exact.

It's now the UK's leading crisp, thanks in part to ads featuring former England soccer player and all-round nice guy Gary Lineker. Those ads were Mr Glenn's brainchild and he is seen as one of his sector's leading marketers.

But he says that marketing is only half the story. "You have to have the product to back it up," he argues. "If you don't, it's just a way of ensuring that you go broke more quickly."

When the company brought Walkers to the Republic, it was convinced it had the product.

"We have Irish people working for us in Britain who grew up eating Tayto and who say that Walker's taste better," he says.

And he's only half joking. He points out that, while the market here was well developed, it was heavily skewed towards cheese and onion flavour. One of the first thing that PepsiCo did was introduce new flavours.

Pepsi recruited Boyne Valley Foods to get the product to retailers and began its marketing campaign.

Mr Glenn says that Irish people "got" the central pitch of the Lineker ads, which is that "even the nicest guy would nick a crisp". But Pepsi adapted its ads, which have variously starred Ireland internationals Roy Keane and Robbie Keane (who luckily featured in Walker's World Cup campaign) and footballers Daragh Ó Sé of Kerry and Ciarán Whelan of Dublin.

Most global brands rely on "one-size-fits-all" advertising. In Ireland's case, Mr Glenn said it suited the company's purpose to add a local element. "At points in our business, it makes sense to tailor a proposition to a local market, if the costs of tailoring it are quite low," he says.

But Walker's Crisps have a long way to go and he stresses that he does not underestimate the opposition, which still holds over 30 per cent of the market.

While the company has been taking on one established brand, it is also selling another, 7Up. Not only is it the leader in its sector, but its penetration is higher here than anywhere else.

"One of the reasons for our great strength in carbonated drinks here has been 7Up. There's a bit of accidental history, when you're a kid growing up in Ireland, 7Up is one of the things that you're given."

The Republic is also one of the strongest markets outside the US for Pepsi.

An unusual aspect of this is that C&C distributes both while fighting to retain Tayto's market share. Both companies are tactful about this clash of relationships, and you can only surmise that it suits everyone.

Mr Glenn explains that, for practical reasons, there are no real synergies between soft drinks and crisps. "The reason we can manage to be both partner and competitor with C&C is that the businesses are run totally separately," he says. Pepsi houses the soft drinks and crisps divisions in different buildings.

But both companies will separately and together have to face up to the fact that many influential people regard the products they sell in the same light as cigarettes. Obesity has become the new public health bogey man, and snack foods are seen as his close allies.

A national task force on obesity is due to report soon. An appearance before the UK equivalent of an Oireachtas committee gave Mr Glenn a taste of the political agendas in this debate. He says that it is a perfect issue for people of the "stupid white man, no logo" persuasion, who blame US companies.

But he believes it's a real issue that is not going away. Nor, he says, can the food industry take the "we just make it" attitude that it has in the past. But it alone should not carry the can.

"It's clearly a problem of modern lifestyle, and that includes food."

He points to a recent initiative in the US, where people are being encouraged to increase their exercise moderately while cutting out 100 calories a day, as a possible model for the way forward.

He insists that more extreme measures won't work. "Obesity hasn't happened because people are opening a bag of crisps, it's 2 per cent of somebody's calorific intake," he says.

"In fact, junk food as it's called is a pretty small percentage overall, so it would be illogical and disproportionate and basically wrong to say that we'll solve the obesity problem by shoving a tax on junk food and banning advertising.

"In Sweden there's been a total ban on food advertising to kids for a number of years, and in Quebec there's a similar thing. Yet in both places the rate of growth in childhood obesity is the same."

The task force could well report by St Patrick's Day, which Mr Glenn will no doubt mark by eating the two bags of crisps he eats every day. By the way, he's definitely not obese.