St Patrick's Day, on March 17th, is an annual celebration of all things Irish – and of one thing in particular. Around Ireland, and all over the world, people celebrate with a pint or two (or three, or four) of Guinness, Ireland's unofficial national intoxicant. Publicans love St Patrick's Day, so much so that it can sometimes feel like less a celebration of Irish culture than a marketing event for Guinness’s owner, Diageo. Now exported to more than 120 countries, the black stuff has become a powerful symbol of Ireland. But how Irish is it really?

Arthur Guinness, who founded a brewery in Dublin in 1759, might have been surprised that his drink would one day become such a potent national symbol. He was a committed unionist and opponent of Irish nationalism; before the Irish Rebellion of 1798 he was even accused of spying for the British authorities. His descendants continued to support unionism passionately – in 1913, one gave the Ulster Volunteer Force £10,000 (worth about £1 million, or $1.4 million, in today’s money) to fund a paramilitary campaign to resist Ireland being given legislative independence. The company was alleged to have lent men and equipment to the British army to help crush Irish rebels during the Easter Rising of 1916, afterwards firing members of staff whom it believed to have Irish-nationalist sympathies.

The beer the company has become most famous for – porter stout – was based on a London ale, a favourite of the street porters of Covent Garden and Billingsgate markets. Since 1886 the firm’s shares have been traded on the London Stock Exchange, and the company moved its headquarters to London in 1932, where it has been based ever since (it merged with Grand Metropolitan and renamed itself Diageo in 1997). As recently as the 1980s, the company has even considered disassociating itself from its Irish heritage. Worried about the impact on sales of the IRA’s terrorist campaign during the Troubles, Guinness came close in 1982 to re-launching the brand as an English beer brewed in west London. But as Northern Ireland’s situation improved in the 1990s, the company’s marketing strategy changed again towards marketing the beer as Irish, aiming its product at tourists in Ireland and the estimated 70 million people of Irish descent living around the world. Now the Guinness Storehouse, part of the original Dublin factory which was reopened as a tourist attraction in 2000, promotes Guinness to tourists as an Irish beer once again.

Guinness is not the only company to play up or hide its national origins in an effort to boost sales. Jacob’s biscuits have been marketed by some shops as being British, in spite of the company’s origins as an Irish company from Waterford. And Lipton now markets its black tea on the strength of the company’s British origins, in over 100 countries – except Britain, where it is not widely sold. In a world where multinational companies control a large chunk of the global food supply chain, national identity – at least in branding – matters as much as ever.