What do Mark Twain, Neil Armstrong and John McCain all have in common?

They’re descendants of Scots-Irish braggarts who crossed the Atlantic from the north of Ireland in the early 1700s and settled in America’s south, giving rise to NASCAR, at least 14 presidents, decisive victories in the Revolutionary War, a third of today’s US military, the NRA, country music, the horror story, American-style democracy, the religious right and victory in the space race.

“The Other Irish” shines a light on...