The difference, by the numbers
They have no ingredient in common: the Irish Coffee has Irish whiskey, coffee, sugar and cream, the Seven and Seven American blended whiskey and lemon-lime soda. In parts, an Irish Coffee is 1 whiskey, 2½ coffee and 1 cream; a Seven and Seven is 1 whiskey and 3½ lemon-lime soda. By volume, the Irish Coffee is 75% mixer and juice, 22% spirit and 3% syrup and sugar; the Seven and Seven is 77% mixer and juice and 23% spirit. Different bottles, similar balance: that makes them close cousins on the ratio map.
They are about equally strong in the glass (both 9.0% ABV), but the Irish Coffee is the bigger drink, so it holds more alcohol (1.13 standard drinks against 1.01). The Irish Coffee has more calories (about 302 against 163), but the Seven and Seven has more sugar (16.2 g against 5.4 g). Both are built; the Irish Coffee is served in an Irish coffee glass, the Seven and Seven in a highball.