The phrase Kathleen Mavourneen is used attributively of something that may not have an end for years, if ever.

This phrase refers to the line “It may be for years, and it may be for ever” in Kathleen Mavourneen, a song set in Ireland, by the English composer Frederick Crouch (1808-1896), with words by a certain Mrs. Crawford. (Mavourneen is from Irish mo mhuirnín, meaning my darling.)

These are the words of the song, as published in The Metropolitan Magazine (London: Saunders and Otley) of September 1835:

Kathleen Mavourneen.
By Mrs. Crawford.

Kathleen Mavourneen! the gray dawn is breaking,
The horn of the hunter is heard on the hill,
The lark from her light wing the bright dew is shaking,
Kathleen Mavourneen! what, slumbering still!
Oh! hast thou forgotten how soon we must sever?
Oh! hast thou forgotten this day we must part?
It may be for years, and it may be for ever,
Oh! why art thou silent, thou voice of my heart?

Kathleen Mavourneen! awake from thy slumbers;
The blue mountains glow in the sun’s golden light;
Ah! where is the spell that once hung on my numbers?
Arise in thy beauty, thou star of my night!
Mavourneen, Mavourneen, my sad tears are falling,
To think that from Erin and thee I must part;
Mavourneen, Mavourneen, thy lover is calling,
Oh! why art thou silent, thou voice of my heart?

An early metaphorical use of the line “It may be for years, and it may be for ever” occurs in the following from The Sun (London, England) of Tuesday 25^th May 1852:

Protection has, at length, been formally abandoned by the leader of her Majesty’s Government. The Earl of Derby ¹, in fact, gave “the go by” even to the five-shilling duty last evening. As nothing, we feel assured, could be more satisfactory, so also we are persuaded nothing could be more explicit than the Noble Lord’s declaration that the project of reviving even a comparatively small impost upon foreign corn, has become at the present day glaringly and absolutely impracticable. […] The Earl of Derby has warned the agriculturists that […] they must make up their minds to endure the fiery ordeal of competition, not for a while, but in perpetuity, to bid farewell to the joys of monopoly, “it may be for years and it may be for ever,” like that memorable early riser, the lover of Kathleen Mavourneen.

¹ The British statesman Edward Smith-Stanley (1799-1869), 14^th Earl of Derby, was then the Prime Minister of the United Kingdom.

The phrase Kathleen Mavourneen has especially been used of a loan that the borrower refuses to pay back. The earliest occurrence that I have found is—as Kathleen Mavourneen loan—from the column Folly Shots, published in The Brooklyn Daily Eagle (Brooklyn, New York, USA) of Sunday 18^th June 1882:

A debtor who was sued by his creditor acknowledged that he had borrowed the money, but declared that the plaintiff knew at the time that it was a Kathleen Mavourneen loan.
“A Kathleen Mavourneen loan,” repeated the court with a puzzled look.
“That’s it, judge; one of the ‘it may be for years and it may be forever’ sort.”

The paragraph first published in The Brooklyn Daily Eagle of 18^th June 1882 was reprinted in many U.S. and British newspapers and magazines, which probably popularised Kathleen Mavourneen loan. This may be the origin of the following paragraph, published in The Lichfield Mercury (Lichfield, Staffordshire, England) of Friday 25^th August 1882—reprinted from the magazine Funny Folks (London, England):

Expressive.—The Americans call money lent which “it may be for years, and it may be for ever” before it comes back, “Kathleen Mavourneen loans.”

The earliest occurrence that I have found of the synonymous phrase Kathleen Mavourneen system is from The Midland Daily Telegraph (Coventry, Warwickshire, England) of Thursday 23^rd June 1892:

Pat’s Promise to Pay.—A son of the Emerald Isle, in the County Court, was sued for the recovery of a small debt, but refused to pay. The judge asked him upon what grounds he objected, when the following conversation took place between the judge and Pat: “Shure, your worship, it was a verbal agreement,” says Pat. “I don’t understand you,” replied the judge. “Well,” says Pat, “it was after we’d struck the bargain, I says, says I, ‘Mr. Moriarty, I’ll be after owing yer sich a trifle’ and he laughed and said ‘All right, Patsy,’ and when I thought he’d gone a convanient distance I shouted out, ‘On the Kathleen Mavourneen system, Mr. Moriarty,’ and he nodded his head and shouted, ‘All right, Patsy,’ and sure it’s not a man that can convince me the time’s arrived yet.” “But you admit owing the money?” “Oh, certainly, yer worship, wid the greatest of pleasure.” “Then you must pay it.” “And so I will, when it’s due.” “What do you mean by the Kathleen Mavourneen system?” said the judge. “Shure, when he was leaving me I sung out, ‘It may be for years, and it may be for ever.’” Then, looking straight at the judge, and sticking his hat on one side, he closed one eye, and said: “And faith it’s for ever, yer worship.”

The phrase Kathleen Mavourneen came to be applied to hire purchase, a system by which one pays for a thing in regular instalments while having the use of it ². This use of the phrase was explained in the following advertisement, published in The Sunderland Echo and Shipping Gazette (Sunderland, County Durham, England) of Friday 26^th April 1929:

It May be for Years
What has this to do with Furniture?

When the Hire Purchase System was first popularized traders had many experiences of defaulting clients, so the system was humorously dubbed the “Kathleen Mavourneen.” In those days the quality of goods supplied was indifferent, to say the least.
To-day conditions have altered. The Hire Purchase client is as keenly catered for as the cash, and the quality of the goods bought on the System must be as high as those purchased for cash.
We have altered the words of the song to “It must be for years.” This applies to the life of the goods supplied on Hire. Before deciding on your purchases inspect our Showrooms.

OUR PRICES ARE LOWEST POSSIBLE FOR CASH.
WE ARRANGE TERMS TO SUIT YOUR INCOME.
Three-Piece Chesterfields in Moquette from
£11 – 10-0
Make Certain—Buy Furniture with a Guarantee at
I. SHARE

² Cf. also ‘glad and sorry’ (i.e., hire purchase).