Lewis Spence said this quote

On the conversion of the European tribes to Christianity the ancient pagan worship was by no means incontinently abandoned. So wholesale had been the conversion of many peoples, whose chiefs or rulers had accepted the new faith on their behalf in a summary manner, that it would be absurd to suppose that any, general acquiescence in the new gospel immediately took place. Indeed, the old beliefs lurked in many neighbourhoods, and even a renaissance of some of them occurred in more than one area. Little by little, however, the Church succeeded in rooting out the public worship of the old pagan deities, but it found it quite impossible to effect an entire reversion of pagan ways, and in the end compromised by exalting the ancient deities to the position of saints in its calendar, either officially, or by usage. In the popular mind, however, these remained as the fairies of woodland and stream, whose worship in a broken-down form still flourished at wayside wells and forest shrines. The Matres, or Mother gods, particularly those of Celtic France and Ireland, the former of which had come to be Romanized, became the bonnes dames of folklore, while the dusii and pilosi, or hairy house-sprites, were so commonly paid tribute that the Church introduced a special question concerning them into its catechism of persons suspected of pagan practice. Nevertheless, the Roman Church, at a somewhat later era, reversed its older and more catholic policy, and sternly set its face against the cultus of paganism in Europe, stigmatizing the several kinds of spirits and derelict gods who were the objects of its worship as demons and devils, whom mankind must eschew with the most pious care if it were to avoid damnation.

~ Lewis Spence

It must not be thought, however, that in pagan Ireland Fairyland was altogether conceived as a Hades or place of the dead. We have already seen that in some of its types and aspects it was inherently nothing of the sort; as when, for example, it came to be confused with the Land of the Gods. In all likelihood these separate paradises and deadlands of a nature so various were the result of the stratified beliefs of successive races dwelling in the same region. A conquering race would scarcely credit that its heroes would, after death, betake themselves to the deadland of the beaten and enslaved aborigines. The gods of vanquished races might be conceived as presiding over spheres of the dead for which their victors would have nothing but contempt, and which, because of that very contempt, might come to be conceived as hells or places of a debased and grovelling kind, pestiferous regions which only the spirits of despised natives or the undesirable might inhabit.

~ Lewis Spence

Here it is necessary briefly to consider the question of the cult of ancestors before venturing farther. The spirits of the departed are believed to be possessed of supernatural powers which they did not enjoy in the flesh. They may also be dissatisfied or malignant in consequence of being suddenly deprived of life, and if they are neglected by the living, are apt to be revengeful. Therefore they must be cajoled and propitiated. Fear of beings belonging to a mysterious state or sphere of which he knew nothing continually haunted and terrified primitive man and induced in him what is known as the dread of the sacred. It was every man's personal duty to attend to the demands or requirements of his deceased ancestors. At first he would succour his own immediate forebears with food and gifts; but it must have been borne in upon him that when his parents joined the great majority, the care of the spirits of their parents likewise devolved upon him... and, by degrees, he might even come to regard himself as responsible for the well-being of a line of spirit ancestors of quite formidable genealogy. These, through his neglect, might starve in their tombs; or, alternatively, they might crave his company. Because of vengeance or loneliness they might send disease upon him, for the savage almost invariably believes illness to be brought about by the action of jealous or neglected ancestors. The loneliness of the spirit-world is the dead man's greatest excuse for desiring the company of his descendants.

~ Lewis Spence

But I find it necessary to repeat in this particular place that the division into classes, which is so salient a part of modern demonology, had, and has, little significance for primitive man or for the peasant in a comparatively low state of mental development. To such people, spirits of all kinds - fairies, the ghosts of the dead, and even witches and water-kelpies - are all creatures of the supernatural class between which he scarcely differentiates.

~ Lewis Spence