Picture stepping off a Boeing 314 Clipper flying boat in County Clare in the early 1940s after some 15 hours in an unpressurized, freezing cabin. At the Foynes air base, about 15 kilometers from today’s Shannon Airport, head chef Joe Sheridan handed battered transatlantic passengers a hot glass of coffee, Irish whiskey, sugar, and chilled cream. The coffee woke them, the whiskey warmed them, the sugar gave quick energy, and the cold cream let them drink it immediately. The episode explains why the sugar is structural: dissolving at least a level teaspoon raises the density of the coffee so the cream can float, and pouring over the back of a spoon keeps it from punching through.

Pulitzer Prize winning San Francisco Chronicle travel writer Stanton Delaplane tasted it at Shannon and in 1952 brought it to the Buena Vista Cafe in San Francisco, where the team spent long nights pouring failed cups down the drain before cracking the float. The drink exploded across America, and Sheridan himself emigrated and went to work at the Buena Vista. As bars cut corners, the National Standards Authority of Ireland published Irish Standard IS 417 in 1988, only to cancel it by 2020. Since then the recipe has become a template for endless variations.

• The 19th century French gloria, a mix of coffee and spirits, appears in Balzac and Flaubert.
• Spain’s version is layered into three separate bands of whiskey, coffee, and cream using special devices.
• In Hanoi, Ethan Haw’s Bar serves iced versions, sometimes with no cream at all.
• Swapping the spirit gives Jamaican coffee with rum, Highland or Gaelic coffee with Scotch, Russian with vodka, and Mexican with tequila.
• New York’s Dead Rabbit bar has served Irish coffee frozen and merged with a martini or a brandy Alexander.