“Let her go, Gallagher!” was a nationally famous slang phrase in 1885 and 1886. Origin is uncertain. “LET her go, Gallagher!” was printed in The Courier-Journal (Louisville, KY) on March 1, 1885, and “LET HER GO, GALLAGHER” was printed in The Courier-Journal on March 10, 1885.
 
A New Orleans, Louisiana, origin was claimed in the The Telegraph and Messenger (Macon, GA) on March 27, 1885, and the Knoxville (TN) Daily Journal on June 16, 1885. A street car driver was called Gallagher, and “Let her go, Gallagher!” from passengers meant that the car was full and that Gallagher should start the ride.
 
Another explanation in September 1886 stated that “Let her go, Gallagher!” referred to town marshal Bob Gallagher of Harrodsburg, Kentucky, and people’s urging him to use his pistol.
 
Another explanation in May 1887, published in the New York (NY) Sun and reprinted in the Chicago (IL) Tribune on May 29, 1887, stated that billiard player and St. Louis (MO) Globe-Democrat turf reporter Tom Gallagher was playing at Mussey’s billiard room in St. Louis, took a long time to shoot, and a spectator yelled “Let ‘er go, Gallagher!” However, this is first cited in print in 1887, and the expression existed since at least 1885. The St. Louis Globe Democrat records Tom Gallagher playing at Mussey’s since 1879, but the saying was not cited in print. The Globe-Democrat reprinted the New York Sun story on June 7, 1887.
 
 
1 March 1885, The Courier-Journal (Louisville, KY), pg. 11, col. 3:
LET her go, Gallagher!
 
10 March 1885, The Courier-Journal (Louisville, KY), pg. 3, col. 4:
LET HER GO, GALLAGHER.
 
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27 March 1885, The Telegraph and Messenger (Macon, GA), pg. 6, col. 4:
“Let Her Go, Gallagher.”
A New Orleans letter says: Every street car driver is called “Gallagher.” The only slang expression in the place is that which is addressed in a chorus to the driver, after a woman has alighted, by the men passengers. She steps on the car, and the driver sits comfortably ahead looking at the mule. He displays every inclination and intention of sitting there forever, until three or four passengers raise their voices and cry, gravely: “Let her go, Gallagher.” Upon this he lets go the brake, and the mule trots daintily on its blithesome way.
 
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16 June 1885, Knoxville (TN) Daily Journal, pg. 2, col. 3:
STREET CAR ETIQUETTE,
As It Is Observed by the Inhabitants of the Crescent City.
(Blakely Hall in Philadelphia Press.)
(...)
I have not been able thus far to find out why every driver is familiarly addressed as “Gallagher,” though I have been told that it is founded upon a legend of ancient date. It would seem that in the early days of the city an Irish policeman—even at that early date the nation had a penchant for getting on the force—named Gallagher arrested a woman and dragged her off to jail. She enjoyed a large degree of popularity and her neighbors followed the minion of the law and chanted: “Let her go, Gallagher,” even to the doors of the prison. How it came to be applied to the drivers of street cars is easy enough to understand.
 
2 September 1886, Detroit (MI) Evening Journal, pg. 1, col. 1:
LET HER GO, GALLAGHER
HOW SLANG DEVELOPS AND GROWS POPULAR
This Particular Piece of Slang Came From a Tragedy—Uncle Phil Armour in Clover—Liberal Offers.
St. Louis Spectator.
I find myself in the position to contribute something to the current literature in regard to slang phrases. In the Sunday Sayings of last week there is a paragraph to this effect: “I have wondered where the new slang phrase, ‘Let her go, Gallagher,’ and ‘Chippie, get your hair cut,’ originated.” They are not particularly bright or meaning on a casual hearing, and I am at a loss to account for the universality of their usage.
   
The first phrase, “Let her go, Gallagher,” I first heard in Kentucky several years ago, and have marked its gradual spreading. The circumstances under which it originated were these: In Harrodsburg, Ky., there was a terrible bully and desperado named Bob Gallagher, who had at different times killed some six of eight men, but under such circumstances that he always escaped punishment upon the plea of self-defense. In his broils he was usually abetted by his two sons, and the trio were a terror in the community. During the period of lawlessness which succeeded the war, he applied for the position of town marshal or policeman. The authorities thought it might prove a good thing, as he and his sons would be held in check, to some extent, and they needed just such a person to deal with the unruly element which was causing so much trouble. He was sworn in and with good effect for a time.  He inaugurated a perfect reign of terror, and for several years it worked like a charm!
 
Finally one day some turbulent spirits came in from a neighboring county, and under the influence of liquor there was a row which Gallagher and his two sons failed to subdue with the accustomed liberal use of their clubs.  Pulling out a pistol he threatened to shoot; quick as thought the other fellow—Hanks, I believe, was his name—pulled his pistol and snapped it so close to Gallagher that it burned the button on his vest, shouting as he did so: “Let her go, Gallagher.” Gallagher did “let her go,” and killed Hanks instantly; he and his sons then fired a number of shots into the dead body of his victim. The murder and trial by which he was acquitted created such a sensation at the time, and he account of it was spread far and wide, coupled with the phrase, “Let her go, Gallagher,” which has since become a by-word.
 
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7 October 1886, The Champion (McPherson, KS), pg. 2, col. 5:
“Let her go, Gallagher,” is a New Orleans product, and is gotten off on drivers of street cars, who are called “Gallagher.” Visitors to the late exhibition in the creole city were struck with the number of that family engaged in driving street cars. There is a legend to the effect that the driver of the first street car run in New Orleans was called Gallagher, which probably accounts for the origin of the gag.—N. O. Picayune.
 
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29 May 1887, Chicago (IL) Tribune, pg. 16, cols. 4-5:
Mr. William Kepley of Chicago Sheds Light on an Important Philological Subject.
“Let her go, Gallagher!” said Mr. William Kepley of Chicago, as he sent a lignum vitae ball down one of the bowling alleys at the White Elephant the other night. He scored a strike. He was asked why he made use of the expression, “Let her go, Gallagher!”
 
“It’s sure good luck,” her said. “It’s better than meeting a man with a straight tip before the races. I saw by the papers out West last summer that it was all the go on ‘Change here. And I saw a lot of funny stories as to where it started. All wrong, every one of ‘em. One said it came from a New Orleans car-driver, another gave credit to a Chicago roller-coaster, and others said that it began in Troy and Baltimore. The Gallagher who gave rise to it was Thomas J. Gallagher, formerly sporting editor of the St. Louis Globe-Democrat, now of THE CHICAGO TRIBUNE. He is well known all over the West. He is one of the best horse reporters in the country. he used to be a first-class sprinter, and he is a fine billiard player. George SLosson says there are only two or three cushion-carrom players besides Schaefer and the other big ones who can beat Tom.
 
“Tom Gallagher used to play a good many billiard matches in Mussey’s old rooms at Fourth and Pine street, St. Louis. When he was practicing a crowd would always be on hand. They all knew him and liked him. Now, Tom had a deliberate way of walking around the billiard table and viewing a hard shot from various points, and then fiddling with his cue reflectively before he shoots, not nervous like, but it’s his way. So when the crowd got tired of waiting some one would call out, as Tom sawed with his cue, “O, let her go, Gallagher!” Finally it got to be a common phrase around St. Louis. Then it got into the pool-rooms and on the race courses. Afterwards the billiard players with whom Tom had matches carried it away with them—Maggioli to New Orleans, Carter to Cleveland, Lon Morris to the coast, and so on until it worked its way East tow years ago and struck ‘Change here contagiously last summer.
 
“That’s all there is to ‘Let her go, Gallagher!’”—New York Sun.
 
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24 July 1887, The Times (Philadelphia, PA), pg. 10, col. 2:
XMAS.—The phrase “Let her go, Gallagher,” has as many alleged authors as the poem of Beautiful Snow. New Orleans, among other places, claims to have furnished the occasion of its first use, somewhat as follows: A street railway of that city whose cars are not started until they will hold no more passengers had a driver named Gallagher. As each passenger would get in he would say, “The car is full, let her go, Gallagher.” This account of the origin of the phrase is probably as good as any other.
 
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21 August 1887, Chicago (IL) Tribune, pg. 20, col. 7:
Who Gallagher Is.
(Poem about Tom Gallagher and billiards.—ed.)
 
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29 April 1889, Pittsburgh (PA) Post, pg. 6, col. 1:
LET ‘ER GO, GALLAGHER.
Some Points About the Old-
Time Billiard Player.
(...)
The famous expression of “let ‘er go Gallagher” was also inspired by Tom, One time in St. Louis he was playing a match, and it got down to a close stage where the whole thing hinged on one shot. Gallagher, who never had a finished stroke, and lacked the dash of Schaefer, was studying the best way to make the play, when an impatient spectator shouted “Let ‘er go Gallagher.” The papers took it up, and from that time on it has become a fixture in the English we wrestle with.
 
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26 April 1935, Potter (NE) Review, “Famous, and Forgotten” by Elmo Scott Watson, pg. 6, col. 1:
Have you ever shouted: “Let ‘er go, Gallagher!” when you were ready to start on a swift ride? If so, you’re only repeating the words of Judge Beaver of Morgan County, Kentucky, during a match trotting race in Tipton county. His fast little mare was being driven by City Marshal Gallagher of Harrodsburg, and at the end of the first half mile the two horses were neck and neck. “Let ‘er go, Gallagher!” shouted the judge when he noticed that the marshal seemed to be holding his entry in. So Gallagher ddid and the little mare won by almost a dozen lengths.