Huntsmen still use stirrup cup to designate an alcoholic drink offered to riders either as they are about to depart or when they return.

Mr. Barry Puilan, Master of the East Antrim Hounds, hands a stirrup cup to huntsman Jack Taylor during the meet at Trench Hill, Ballyeaston, yesterday.

from The Northern Whig and Belfast Post (Ireland) – 13^th January 1953

The term stirrup cup originally denoted a cup of wine, or of other alcoholic beverage, offered to a horseman ready to ride away, hence also any farewell drink.

One of its earliest occurrences is found in The Praise of York-shire Ale, wherein is enumerated several sorts of Drinks, with a Discription [sic] of the Humours of most sorts of Drunkards (3^rd edition – 1697), by George Meriton (1634-1711?), a legal and miscellaneous writer:

     (1851 reprint)
Boy, lead our horses out; when we get up,
Wee’l have with you a merry stirrup cupp;
Then we to famous Yorke will hast away,
For thither wee’l adjourn the court this day.
The horses were lead out, they mounted all,
And each of them did for a flagon call.

The expression also denotes a drink offered to an arriving guest before he has dismounted, and the drinking vessel itself; the following is from The Illustrated London News of July 1982:

SALEROOMS

Silver belonging to an aristocratic Russian family who fled the Revolution comes up at Phillips on July 16. It includes Fabergé carved animals, silver gilt and niello beakers, cutlery, an 18^th-century plate and a fine stirrup cup in the shape of an eagle’s head.

English stirrup cup corresponds to the obsolete French le vin de l’étrier, literally the stirrup wine, first attested in Le Page disgracié, ou l’on void de vifs caracteres d’hommes de tous temperamens, & de toutes professions (1643), by the French author Tristan l’Hermite (François l’Hermite – circa 1601-1655):

     (1667 edition)
Il s’auisa de […] me dire que […] il me prioit d’aller oüir la Messe auec luy dans vn deuot Monastere, & que nous boirions apres le vin de l’estrié : ie ne luy voulus pas refuser cette faueur, apres en auoir receu d’autres de luy.
     translation:
He ventured to […] tell me that […] he was asking me to go and hear Mass with him in a devout monastery, and that we would afterwards drink the stirrup wine: I did not want to refuse him this favour, after receiving others from him.

The usual French term is now le coup de l’étrier, where coup means quantity of wine, or of other alcoholic beverage, that one can drink in one go.

The modern equivalent of stirrup cup is the phrase one for the road, meaning the final drink before departure. The earliest instance that I have found is from The Northern Whig and Belfast Post (Ireland) of 12^th September 1936 (the fact that one for the road is in quotation marks indicates that it was already well established):

DRINK TESTS ON DRIVERS.
“One For the Road” a Mistake.
TEETOTAL DEMAND.
Scientist’s Address to British Association.

Tests carried out at the National Institute of Industrial Psychology by a committee appointed by the British Medical Association showing the effects of alcohol on motorists were described at the Physiology Section of the British Association at Blackpool yesterday by Dr. H. M. Vernon, of London.
The tests carried out at the request of the Minister of Transport proved conclusively, he said, that it was a mistake to imagine that “one for the road” made no difference to the motorist’s control of the wheel.

RULE IN USE.

“It is most desirable,” Dr. Vernon said, “that the motorists should not drink any alcohol at all before driving. This rule is already followed by the vast majority of the drivers of public conveyances.
“Unfortunately the drivers of private cars frequently do not follow it. Recent tests have shown that in many cases persons involved in traffic accidents had considerable quantities of alcohol in their blood.
“Medical tests indicate that all persons without exception are definitely under influence of alcohol when they have as much as two parts per thousand in their blood, while about half of them are under the influence when they have one part per thousand.
Dr. Vernon said the experiments at the National Institute of Industrial Psychology were carried out with a dummy car in artificial conditions closely approximating to road driving. They showed that while a quarter of a pint of very mild beer had no appreciable effect on the driver’s capabilities two to four ounces of whisky increased the speed of driving by six per cent. on the average. At the same time 12 per cent. more errors were made. Twenty drivers were tested with considerably varied results.
Another doctor said that if he were the Minister of Transport he would say that every person under thirty should be teetotal when driving a motor car.
A young woman related that after she had had a glass of cider—not knowing that it was alcoholic—she could not get off the middle of the road, and could not change gear.

One for the Road (1984) is a play by the English playwright, actor and director Harold Pinter (1930-2008).

The French equivalent phrase is un, or le, dernier pour la route, that is, a, or the, last one for the road.

According to the New-Zealand born philologist Sidney John Baker (1912-76) in Australia speaks: a supplement to “The Australian language” (Sydney – 1953), the colloquial Australian equivalent of one for the road is one for the bitumen, the bitumen being used to designate a tarred road, specifically the road from Darwin to Alice Springs.

FOLK ETYMOLOGY

The motorism context of the earliest instances of one for the road clearly shows that the phrase did not originally refer to the supposed practice of offering a last drink to a condemned prisoner about to be taken to, or on his way to, the place of execution.