The phrase to get, or to come, down to brass tacks appeared in American English and means to concern oneself with the essential characteristics of something. It is first recorded in 1863.

I have discovered an earlier American-English phrase, to come (right) down to the brass, which meant:
– to get to the point, i.e., to get to the essential element of something being discussed,
– to concern oneself with the essential characteristics of something.

The earliest instance of this phrase that I have found dates from 1854.

I have also discovered a variant of the latter phrase, to bring [someone, or something] (right) down to the brass, which meant to cause [someone, or something] to reveal their, or its, essential characteristics.

I have not, however, discovered what the brass refers to in those two phrases. (I have presented here their earliest occurrences as well as those of to get, or to come, down to brass tacks.)

As for the origin of to get, or to come, down to brass tacks, it has been the object of much speculation.

An anonymous text titled Brass Tacks, reprinted from the Cincinnati Times (Cincinnati, Ohio) in The Wyandot Pioneer (Upper Sandusky, Ohio) of Thursday 14^th May 1868, perhaps points to an explanation.

In this text, the term brass tacks designates the broad-headed nails of brass studded over a coffin, and as a consequence is suggestive of the fundamental reality that death is ineluctable. Therefore, the phrase that is used in the text, to bring [someone, or something] (right) down to brass tacks, means to cause [someone, or something] to reveal their, or its, essential characteristics, in the same manner as death puts an end to any possibility of dissimulation and artifice.

It is possible, however, that this particular connotation attached to brass tacks is merely idiolectal, i.e., is peculiar to the language of the author of this text, and is therefore not indicative of the origin of the phrases to come (right) down to the brass, to bring [someone, or something] (right) down to the brass and to get, or to come, down to brass tacks.

The following is a transcription of Brass Tacks, as published in The Wyandot Pioneer on 14^th May 1868:

Bring things right down to brass tacks in all the affairs of this life and the millenium [note 1] is not far away. Brass tacks—emblem of the only inevitable and last friend, the undertaker. Studded over our final ligneous adornment, brass tacks are suggestive of stern, inexorable reality; sham and shoddy are no longer available; deceit and pretence are below par. Brass tacks have equalized all human earthly conditions. The peer and peasant, king and common, old and young, wise and otherwise, lie down in a common mortality from which there is no escape. Once before—at birth—they were all equal in ignorance and helplessness. They were not consulted then. They have had many opportunities for good and evil since; they have strutted life’s busy hour upon Time’s stage, playing their allotted parts with more or less earnestness, in farce or tragedy, some to the pit, others to the dress circle—this one applauded, that one hissed, until, again without being consulted, death bring them down to brass tacks.
We begin, in fact, to doubt the reality of everything this side of brass tacks. Shams and shoddy pervade society, and civilization is a mockery and a delusion. “I am a Christian,” says one. Ah! bring it down to brass tacks—which of sixty oppugnant [= antagonistic] and wrangling sects do you fight for? Oh, Catholic, eh—the Latin branch or its irreconcilable enemy, the Greek Church?
The whole world is full of misunderstandings. People do not define things by their right names, and are always in mischief and trouble. Language is employed to conceal thought. One’s face is intended to hide character; no one is what he seems. “Turn to the right!” said a man on one side of a river to another man on the opposite side who had inquired the way to the ford. The querest [misprint for querent] obeyed, and was drowned. “Good gracious! I meant my right, not his; how unfortunate!” Just so through every phase of life—we misname things. We don’t bring them down to brass tacks.  Nothing is what it appears to be on the face of it. All is buncombe and sham, from mock auctions, quack doctors, sensation preachers, acrobat politicians to thimble-riggers of all kinds. We used to bet freely and confidently on human nature, civilization, progress, pretty girls, our preacher and our doctor; but we don’t any more.
The ladies, resplendent in silks and diamonds glowing with the roses of youth, graceful in form, teeth of pearl, eyes of light, hair falling like golden fleece over billowy shoulders of snow. Alas! matrimony brings them down to brass tacks, Kalosina [note 2], cotton, India-rubber, sawdust, crinoline, false hair, plumpers [note 3], calves [note 4], palpitators [note 5], &c., dispel the illusion.
Humbug has come to be one of the polite arts. Quackery is a science. Swill milk and poison whisky are popular lies. Wooden oats tin side saddles, wax buttons, paper collars, glued boots, only illustrate how the world is given to lying.
When the Indian, in pursuit of his enemy, seized him by the hair just as he had plunged into the river and the wig came off, the red gentleman very naturally exclaimed in a tone of indignant disgust, as he held up the tonsorial chef d’œuvre, “d—n lie.”
Nearly every thing is a whopper. We get nothing that we pay for now-a days. We scarcely know what we eat and drink any more than if we lived at a Chinese hotel. The horses pass the bologna-sausage dealers with reproachful looks, and in the ‘sassenger’ [= sausage] season one is almost ashamed to look a well-bred dog in the face.
We saw a few days ago, several casks of terra alba, which, being interpreted, means white clay. Upon inquiring what it was to be used for, we learned to make cream of tartar and sugar ornaments. Our coffee is rye and chickory, sugar is sand, flour plaster of Paris, pepper and alspice black walnut sawdust, oak bark for cinnamon, corn meal for mustard and ginger, blue clay for blue pill, and so on with everything. As to the liquors we imbibe, no language can do justice to the subject. It involves, besides, several great moral questions, a treatise upon poisons and man’s wonderful powers to endure. Bring everything right down to brass tacks, and what becomes of all our pretentious progress? The doctor who has himself called out of half a dozen churches every Sunday morning is like unto the wig—and the best of them when brought face to face with disease and death, and peremptorily required to exhibit their boasted knowledge, are brought down to brass tacks, and must own their helpless ignorance.
The evangelical and model minister, whose creed involves infant damnation, election, resurrection of the material body and triple Deity, when brought down to brass Tacks, disbelieves it all—or, what is the same thing, doubts. The lawyer who rushes to and fro from court with a huge green bag filled with old newspapers, is another wig case.
Politicians, before whose mercenary achievements the famous Esau case of barter and sale of franchises dwindles into mediocrity—a constable’s office buys their hope in this world, and a postoffice their chance of salvation hereafter. When city councils and legislatures require the services of lobby members; when Senators are bought and sold in the market, and have values, discounts and fluctuations on Wall street, like any other property; and impeachment is subject to local politics and private interests—things had better be brought down to brass tacks.
Let us unite in——sighs for the good old days of good neighbors: quiltin’s, and corn huskin’s, apple jack, and spring-water.—Cin. Times.

The following is an illustration for Cripple Buries Body of His Old Friend. Builds Coffin With His Own Hand and Conducts the Funeral Himself, published in The St. Louis Republic (St. Louis, Missouri) of Thursday 22^nd November 1900—James Thornton, an old, one-legged soldier, is building the coffin of his friend, Oliver Turner:

When at last the boards had been fashioned and fitted into the crude semblance of a coffin, Thornton […] exhibited several lengths of cloth. It was cheap, but it was black. With brass-headed tacks  he nailed it on, and on the lid made a faint effort at a studded design.
It was ten minutes to 4 o’clock in the afternoon when the coffin was finished and the old gray mule was driven around into the alley. Thornton let down the end-gate of the wagon, the black-covered box, studded with the brass-headed tacks, was lifted in, and the journey to the morgue was made.

 

Note 1: The correct spelling is millennium, not millenium. In Christian theology, the millennium designates the prophesied thousand-year reign of Christ at the end of the age. By extension, the millennium denotes a utopian period of justice, peace and prosperity.

Note 2: I have not discovered the meaning of the word Kalosina.

Note 3: The noun plumpers denotes small balls or pads carried in the mouth to fill out the cheeks artificially.

Note 4: Here, calves denotes artificial calves, as explained in an article titled About False Calves, published in The Round Table (New York City, N.Y.) of Saturday 3^rd March 1866:

During several weeks of the past winter a thriving business was done by the corset manufactories in the production of artificial calves. And by calves we mean just what the anatomists mean when they speak of the lower extremities. We do not know whose ingenuity devised them, nor when they were first introduced, nor, indeed, the method of their construction. But that they were a very popular article of apparel with young ladies, and especially those who made dashing displays on skates, we have abundant reason to believe. In fact, several of the prominent corset-makers devoted all their energies to the fabrication of these rare bits of fashionable anatomy […].
[…]
[…] In fact, the rage of the season has been these adjustable calves, nor has the demand fallen off very materially with the passing away of the skating season. They are worn by the most fashionable, if not the most respectable, in the daily promenade and at the weekly social gathering, and, in fact, almost everywhere. Very nice young men stand behind counters all day long and sell them to very nice young ladies in sizes to suit. So it makes little difference how cadaverous or ill-shapen one may be, even nature is outdone by the devices of art. What with an investment or two in false hair, a false bust, plumpers in the cheeks, and the now thoroughly introduced patent calves, the most awkward in shape and unattractive in general appearance may become really “charming.”

Note 5: The Wheeling Daily Intelligencer (Wheeling, West Virginia) of Wednesday 4^th July 1866 defined the noun palpitator:

The last and most ingenious device invented for the purpose of enabling the ladies to delude the men by deceitful show is called a palpitator. It is an artificial bosom, made with a steel frame, which encloses a bellows, operated by clock work. The machinery gives that portion of the bosom which is intended to be looked upon a gentle heaving motion about twenty times a minute. The effect is said to be wonderful. The palpitator may be readily attached to that garment worn to give shape to the body, while it gives it support by the aid of drilling, whalebone, eyelets, lacing, &c.