The concept of the “sin-eater” is one of those curious ideas that strikes such a chord in people that it gains currency and becomes a commonplace in popular culture without ever really being the subject of proper scholarly enquiry.

We can say, however, that the evidence concerning sin-eating was first investigated and analysed by a folklorist by the name of E. Sidney Hartland, who published a paper on the subject in the journal Folklore, and that the practice apparently died out in the late 19th or very early 20th centuries. Hartland thought in 1892 that sin-eating was by then extinct, but had been “formerly observed in Wales and the Welsh marches” (the borderlands between England and Wales, comprising the western parts of Cheshire, Shropshire, Herefordshire and Gloucestershire, roughly from Chester to Gloucester), typically taking place as part of the rituals surrounding funerals.

The earliest source for the practice that Hartland found was the diarist John Aubrey’s folklore miscellany “The Remaines of Gentilisme and Judaisme, 1686-87,” which, although written in the 17th century, was not published until 1881. I have not been able to trace published discussions of sin-eating to much before this latter date, which makes it very difficult to know how widespread, common, or varied the practice was before the late 19th century, when it was already practically extinct. If Aubrey’s evidence is to be believed, it was already fading away even in his day, for he called it “an old Custome” and wrote of it in the past tense.

According to Aubrey, it had been customary in Herefordshire

at funerals to hire poor people, who were to take upon them all the sinnes of the party deceased. One of them I remember lived in a Cottage on Rosse-high way. (He was a long, lean, ugly, lamentable Raskel.) The manner was that when the Corps was brought out of the house, and layd on the Biere; a Loafe of bread was brought out, and delivered to the Sinne-eater over the Corps, and also a Mazar-bowl of maple (Gossips bowle) full of beer, which he was to drinke up, and sixpence in money, in consideration whereof he took upon him (ipso facto) all the Sinnes of the Defunct, and freed him (or her) from walking after they were dead.

Aubrey further noted that the custom was “rarely used in our dayes” but had been observed in some places even at the height of the religious restrictions imposed by the Cromwellian government in the 1650s. He had heard of instances which took place in Hereford itself, in Brecon in South Wales, “where Mr. Gwin the minister about 1640 could no[t] hinder ye performing of this ancient custome,” and indeed all over Wales. In North Wales, he added, sin-eaters drank a bowl of milk, not beer.

At least one other account from Herefordshrie does exist; Ella Mary Leather, who collected folk stories from the area in the early 20th century, was told by a guest at a farmhouse wake that when he had declined a glass of port, the old farmer told him: "But you must drink, sir. It is like the Sacrament. It is to kill the sins of my sister." And the discovery of Aubrey’s account prompted other folklorists to recall evidence of similar practices that had taken place elsewhere in Britain. A gloss to Aubrey’s manuscript by “Dr. Kennett” – I believe this to be the antiquary White Kennett (1660-1728), Bishop of Peterborough, who was vicar of Ambrosden, Oxfordshire, from 1685-1703, and is known to have come into the possession of some of Aubrey's manuscripts – added a reference to “Amersden, in the county of Oxford, where at the burial of every corps one Cake and one flagon of Ale just after the interment was brought to the minister in the Ch[urch] porch” – which it seems to me may or may not refer to a related practice. Hartland then notes that further evidence of the practice comes from Shropshire, where a letter – written by a John Bagford, dated February 1714/15, and originally published in the antiquary John Leland’s Collecteana – reports that Bagford had learned from a second Aubrey manuscript – long lost – that

Within the memory of our Fathers, in Shropshire, in those villages adjoining to Wales, when a person dyed, there was a notice given to the old Sire, (for so they called him), who presently repaired to the place where the deceased lay, and stood before the door of the house, when some of the Family came out and furnished him with a Cricket [stool], on which he sat down facing the door. They then gave him a Groat [a coin worth four pence], which he put in his pocket; a Crust of Bread, which he eat; and a full bowle of Ale, which he drank off at a draught. After this, he got up from the Cricket and pronounced, with a composed gesture, the ease and rest of the Soul departed, for which he would pawn his own Soul.

The third and final account collected by Hartland dates to the first half of the nineteenth century and was read by a gentleman antiquary named Matthew Moggridge, of Swansea, at a meeting of the Cambrian Archaeological Association held at Ludlow (also in the Welsh Marches) in 1852. This example came from Llandebie, a parish some miles north Swansea, and was described as a custom that “survived to within a recent period”:

When a person died, the friends sent for a Sin-eater of the district, who on his arrival placed a plate of salt on the breast of the defunct, and upon the salt a piece of bread. He then muttered an incantation over the bread, which he finally ate, thereby eating up all the sins of the deceased. This done, he received his fee of 2s. 6d. and vanished as quickly as possible from the general gaze; for, as it was believed that he really appropriated for his own use and behoof the sins of all those over whom he performed the above ceremony, he was utterly detested in the neighbourhood – regarded as a mere Pariah – as one irredeemably lost.

This is the closest we come to the idea of sin-eaters being “stoned and chased off,” as you suggest; if Moggridge’s account is accurate, it would seem they were hired directly by the family of the deceased and that their services were regarded as an unfortunate necessity. As such, it seems to me unlikely they would ever have been treated with quite the hostility that O'Brian imagines.

Exactly how these customs came into existence is not clear. Napier, writing of the folklore of the west of Scotland in the first half of the 19th century, but publishing his observations after Moggridge's quite similar report appeared (it would be unsafe, in fact, to assume that his account is distinct from Moggridge's, or even relates to practices that took place in Scotland) noted that

There were persons calling themselves "sin eaters" who, when a person died, were sent for to come and eat the sins of the deceased. When they came, their modus operandi was to place a plate of salt and a plate of bread on the breast of the corpse, and repeat a series of incantations, after which they ate the contents of the plates, and so relieved the dead person of such sins as would have kept him hovering around his relations, haunting them with his imperfectly purified spirit, to their great annoyance, and without satisfaction to himself. This form of superstition has evidently a close relation to such forms of ancestor-worship as we know were practised by the ancients, and to which reference has already been made.

But this seems a stretch to me, and Aubrey linked the idea of sin-eating – much more plausibly in my opinion – not to some surviving bit of pre-Christian folklore but to the concept of the scapegoat, described in the Old Testament Book of Leviticus (16:21-22):

And Aaron shall lay both his hands on the head of the live goate and confesse over him all ye iniquities of the children of Israel, and all their transgressions in all their sins, putting them upon the head of the goat, and shall send him away by the hand of a fit man into the wildernesse.

A further possibility, raised by the folklorist Jacqueline Simpson, is that sin-eating was a distorted memory of Catholic funerary practices that had been abolished a century before Aubrey's time, during the Reformation: "What Aubrey describes, well over a hundred years later, appears to be a confused attempt by post-Reformation generations to go on carrying out Catholic traditions." In this interpretation, the "incantations" that the sin-eater supposedly mumbled over the corpse were the payers formerly said at the Requiem Mass, forbidden by the new Protestant theology.