The British- and Irish-English phrase where there’s muck, there’s brass (in early use where there’s muck, there’s luck and where there’s muck, there’s money) means that dirty or unpleasant activities can be lucrative.

The English naturalist and theologian John Ray (1627-1705) recorded a phrase expressing the same idea in A Collection of English Proverbs Digested into a convenient Method for the speedy finding any one upon occasion; with Short Annotations. Whereunto are added Local Proverbs with their Explications, Old Proverbial Rhythmes, Less known or Exotick Proverbial Sentences, and Scottish Proverbs (Cambridge: Printed by John Hayes, Printer to the University, for W. Morden, 1678):

Muck and money go together.
Those that are slovenly and dirty usually grow rich, not they that are nice and curious in their diet, houses and clothes.

These are, in chronological order, the earliest occurrences of where there’s muck, there’s luck and where there’s muck, there’s money that I have found:

1-: From a letter that Dr. Sheridan wrote to Mrs. Whiteway, most probably in November 1735, after Jonathan Swift ¹ broke his shin—as published in Letters, written by Jonathan Swift, D. D. Dean of St. Patrick’s Dublin, and several of his Friends, from the Year 1696 to 1742 (London: Printed for C. Bathurst, W. Strahan, J. and F. Rivington, L. Davis, W. and E. Johnston, W. Owen, R. Baldwin, T. Longman, J. Dodsley, and T. Cadell, 1775):

Dear Madam,
To say the truth, I am not concerned for the Dean’s accident, since my friend Jacob says there is no danger in it; because it keeps him from his long walks, by which means I see he is gathering flesh, and I hope will gather health and wealth by being here; for, as the Scotchman says, Where there is Muck, there is Luck.

¹ Jonathan Swift (1667-1745) was an Irish satirist, poet and Anglican cleric.

2-: From Poems on Several Occasions. By John Bennet, a Journeyman Shoemaker (London: Printed for the Author, 1774):

On seeing some Young Ladies sitting in a Room hung with Cobwebs.

When belles so bright,
Shine with such light,
And cobwebs in the room;
It makes folk think
There must be chink *,
Because no use of broom.
* Alluding to a vulgar proverb, “Where there’s muck there’s money.”

3-: From Statistical survey of the County of Meath, with observations on the means of improvement; drawn up for the consideration, and under the direction of the Dublin Society. By Robert Thompson, of Oatland (Dublin: Printed by Graisberry and Campbell, 1802):

A great deal of the filth, at present to be found about the farm-house, is in a great degree attributable to the tenant, but much more so to the neglect of the landlord, in not providing for him the means of cleanliness. As long as the present mode is continued, in having the foddering yard in front of the farm-house, instead of having it in the rear, every exertion towards cleanliness, in the housewife, will be fruitless; for though a common expression prevails amongst people of this class, that “where there is muck there is luck,” yet I never understood, that muck was of necessity to lie at the front, nor that the farmer and his family should wade knee deep in it, every time they went in or out of the house; the farm-houses may literally be said to be immersed in excrement.

4-: From The British Tourist’s, or, Traveller’s Pocket Companion, through England, Wales, Scotland, and Ireland. Comprehending the most celebrated modern tours in the British Islands, and several originals (London: Printed for Richard Phillips, 1809), by the Scottish teacher, cleric and author William Fordyce Mavor (1758-1837):

Llanidloes, standing in a pretty fertile vale, is rather a neat Welsh town […].
[…] The streets are filthy to an extreme degree; but it would be singular to find a house in Wales without dung-heaps before the door. Whether they believe in the old proverb, “that where there is muck there is money,” I know not, but certain I am, that they act as if they did.

These are the earliest occurrences of the phrase where there’s muck, there’s brass that I have found:

1-: From the Leeds and Yorkshire Mercury (Leeds, Yorkshire, England) of Wednesday 4^th September 1907:

PROPOSED LIGUE OF BEAUTY.
[…]
AN IMMENSELY USEFUL ORGANISATION.
VIEWS OF MR. V. GRAYSON, M.P. ²

I distinctly like the project, and am glad to see that there are some active æsthetes still in existence.
We have all heard of the complacent Alderman who defended the murk of Oldham ³ by the cheery aphorism, “Where there’s muck there’s brass.” It is against this type of profit-grinding Philistine that everyone with artistic perceptions must strenuously fight.

² Victor Grayson (born 1881–disappeared 1920) was a British socialist politician, Member of Parliament for Colne Valley, Yorkshire, from July 1907 to January 1910.
³ Oldham is a town in Lancashire, England.

2-: From the transcript of a speech that Victor Grayson delivered at a meeting of pottery operatives held in Hanley, Staffordshire, published in the Staffordshire Sentinel (Stoke-on-Trent, Staffordshire, England) of Tuesday 17^th September 1907:

Delightful how people used the word “art” nowadays! There was art in the potting business and this was why it was desired that the children in the Potteries should have art. There was an Oldham Councillor who said, sticking his hands into his pockets and looking around at the dreary murk, “Never mind, where there’s muck there’s brass.” (Laughter.) But now employers had risen above that bestial conception, and they said “We will send you to an art school.” (Laughter.)

3-: From a letter to the Editor, published in The Daily Mail (Hull, Yorkshire, England) of Friday 24^th December 1909—in this letter, a Midlander gives his impressions of Hull:

You have some really fine public buildings, well situated and harmonious in aspect, and your main roads and many streets are both wide and admirably laid out. Educational and artistical facilities are in great evidence, as your fine schools, libraries, and art galleries testify. You have many huge hives of industry, giving employment to thousands of workers, and your shipping interest is of world-wide fame. Magnificent shops adorn your principal streets, affording pleasure to the observer, and doubtless satisfaction to their proprietors and patrons. Thus for to the credit side. Unfortunately there is another side to the picture—whether from causes unknown to the writer, or from a too literal acceptance of the adage “Where there’s muck there’s brass,” I cannot say, but I do not thing [sic] for its size have I in all my experience seen such woeful disregard of the cleansing of the highways and footpaths.