There's an excellent commentary on Billy Barlow in Irwin Silber's Songs of the Civil War (1960 - available as a Dover reprint):

BILLY BARLOW. One of the great stock characters of the nineteenth-century stage was the roguish, loud-mouthed braggart whose seif-confidence was matched only by his disreputable appearance and outrageous lies. The personification of this character took many names, but one of the mostt popular was "Billy Barlow." One of the earliest references to "Billy Barlow" in our popular music is 1836, with a version sung by Jack Reeve including such verses as:

I went down the street the other fine day,
Met two fair ladies just coming this way;
Says one, "Now that chap, he isn't so slow,"
"I guess not," says the other, "that's Mr. Barlow."

I'm told there's a show coming into the town,
Red lions and monkeys and porcupines brown;
But if they should show, I'll beat them I know,
For they've never a varmint like Billy Barlow.

I went to the races on Long Island so gay,
The man at the gate, he asked [me] to pay;
"What pay!" says I, and I looked at him so—
"Pass on, sir, I know you; you're Mr. Barlow!"

For the next quarter of a century ragged Billy kept bobbing up on the stage in New York, Philadelphia, Boston, and New Orleans, commenting on the times and blowing his horn. Early in 1861, shortly before the war broke out, Billy Barlow was singing in New York:

Our country's excited 'bout this thing and that,
Both North and the South hardly know what they're at.
They secession, coercion and compromise blow,
But it's talk and no cider—thinks Billy Barlow.

For once, Billy proved himself a poor political prophet. His prediction was belied by the attack on Fort Sumter, but he still managed to produce a couplet which deserves enshrinement along with the best of Mr. Dooley and Will Rogers:

Our members of Congress have plenty to do,
But it's seldom, if ever, they do it, 'tis true.

The version printed in this collection (first message, top of this thread) is a wartime set of lyrics. While the subject matter is military, Billy Barlow is still the same, modestly pointing out in a verse concerning the Union defeat at Bull Run:

It's true they got routed, but then you all know,
It was on account of the absence of Billy Barlow.

But all of the above is only half of the fascinating history of Mr. William Barlow. For Billy turns up in folk song, too, still an unreconstructed, obstreperous fellow in a traditional children's song. And this Billy Barlow traces his lineage back to fourteenth-century England through a score of characters such as John the Red-Nose and Robin the Bobbin.
The melody of the stage song is different, and so is the story, the trace of traditional folk song remaining only in the title and the character of the hero. But there is no doubt that it is, if not the same Billy Barlow, a reasonable and remarkable facsimile thereof—in tune with the times.