MARCH 17th is St Patrick's day, a celebration of all things Irish—and of one thing in particular. Around Ireland and all over the world people will 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 the 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, who before the Irish Rebellion of 1798 was even accused of spying for the British authorities. His descendants continued passionately to support unionism—one giving the Ulster Volunteer Force £10,000 in 1913 (about £1m, or $1.7m, 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....