Following with interest. Will try some of gargoyle's suggestions.

Walt Whitman, best known as a poet, apparently was the firsr to mention shebang in print, 1862-1863, in his Journal entries about the Civil War. "Their shebang enclosures of bushes." 1863: "The soldiers guarding the road came out from their tents or shebangs of bushes."
Bret Harte, 1877, used the phrase "the whole shebang," but Mark Twain used "shebang" to refer to a project (a book) in 1869, and a vehicle in 1872.

Seems to have been a soldier's term for a temporary shelter made of brush; Irish origin speculative at best, may date to the soldiers in the west after the Mexican War.
The OED and Webster's considers the origin to be American.