The firm is spending pounds 7.5m on using St Patrick and his 17 March festival to push its pints. Murphy's plans parties in more than 20,000 pubs and bars across the world a week today - and claims this makes it "Official Sponsors of the St Patrick's Day Celebrations".
Does it? "It's like saying they've sponsored Christmas," said a bemused spokesman for the Catholic Media Office. "Saints are nobody's property."
The abstinence movement, a powerful lobby among Roman Catholics in Britain and Ireland, would be upset by the association of the saint's name with alcohol, he added. "There'll be a lot of people who will celebrate St Patrick's Day who would be opposed to the demon drink."
"Who do they think they are?" asked Father Edward Featherstone, a Roman Catholic priest who also belongs to the Calix Society, a group that aims to help people recovering from addiction. "It is in poor taste."
Murphy's will risk it. Juniors in the stout market, its bold move is part of a continuing campaign to grab sales from the mighty market leader, Guinness, and in particular to follow up the spectacular PR blow it landed during President Clinton's visit to Ireland. Bill drank half a pint of the black stuff in a Dublin pub that usually sells Guinness only - but Murphy's smuggled in a single keg of its stout, from which it claimed Mr Clinton. drank. Guinness denied it, but the damage had been done.
Guinness is no happier that its smaller rival is now trying to follow up the president with the saint. "I'd dearly love to know who gave them the right to say this," said Guinness spokesman Jeremy Probert. "Maybe it was the saint himself, who is 1,500 years old, so he's obviously holding up well."
Guinness would also be spending big money on promotions in the week leading up to St Patrick's Day, he said - not least on sponsoring the Cheltenham Festival. Guinness had long been established as the Irish tipple, he added. "You ask an Irishman in a bar what is the national drink of Ireland and he'll say Guinness. We are very proud of our Irishness."
One million pints of Guinness are drunk in Britain every day. But Murphy's, which is acknowledged as a smoother and less bitter drink, is coming up fast on the rails with 1.4 million a week.
The rivalry began in 1987, when Murphy's, the product of a brewery in Cork, was launched in Britain. Ironically, this was a response to Guinness's success in changing its image. It was already the market leader by far, synonymous with Dublin since Arthur Guinness set up a brewery in 1759 using the soft water of Wicklow mountain rivers. In the early Eighties, it went all out to attract younger drinkers. The serving temperature was dropped, to bring it in line with lagers fashionable at the time, and the firm launched the stylish "Pure Genius" advertisements featuring Rutger Hauer.
Jill Rennie, a spokeswoman man for Murphy's, said it would be sponsoring themed television programmes on St Patrick's Day and advertising on TV, radio and in the press. Free drinks for all were not on the agenda, but there would be themed party nights with prizes, and the chance to win a place at a party to be held in Murphy's brewery in Cork on Sunday.
Annoyed by Guinness's claims to be Ireland's national drink, she asked: "How can they make that claim? Who says so? Guinness try to own everything Irish. We want people to know there are other brands out there."
So which is more truly Irish? Most pints of draught Guinness drunk in Britain originate in the company's giant brewery in Park Royal, north- west London. But then every pint of draught Murphy's drunk here is brewed in Magor in Gwent, south Wales.
The Irish embassy in London refused to adjudicate, other than to say there was no officially registered national drink.
Irish pubs are suddenly fashionable in London; the biggest and flashest of them, Waxy O'Connor's, opened in Soho in October, and attracts huge crowds at weekends, with 300-400 people queuing to get in. Barmen Jack and Anton - both banned from drinking while they work - agreed that Guinness was the archetypal Irish drink and the most-requested pint, but not necessarily the best-tasting.
Assistant manager Martin Jordan liked Murphy's more than Guinness because "it hasn't got the sharp taste". But for a genuine taste of Ireland, he recommended a third name, Beamish, as the only draught stout available that had been brewed exclusively in Ireland, using local waters.
Beamish stout has not been promoted as aggressively as Guinness and Murphy's, but a spokesman for Scottish & Newcastle, which has recently acquired the Beamish & Crawford brewery in Cork, said the company now planned to put its considerable weight behind advertising the brand.Reuse content