But why were the Tuatha De Danann associated with the mounds?  If fairies or an analogous race of beings were already in pagan times connected with hills or mounds, gods now regarded as fairies would be connected with them.  Dr. Joyce and O’Curry think that an older race of aboriginal gods or sid-folk preceded the Tuatha Dea in the mounds.[208] These may have been the Fomorians, the “champions of the sid,” while in Mesca Ulad the Tuatha Dea go to the underground dwellings and speak with the side already there.  We do not know that the fairy creed as such existed in pagan times, but if the side and the Tuatha De Danann were once distinct, they were gradually assimilated.  Thus the Dagda is called “king of the side”; Aed Abrat and his daughters, Fand and Liban, and Labraid, Liban’s husband, are called side, and Manannan is Fand’s consort.[209] Labraid’s island, like the sid of Mider and the land to which women of the side invite Connla, differs but little from the usual divine Elysium, while Mider, one of the side, is associated with the Tuatha De Danann.[210] The side are once said to be female, and are frequently supernatural women who run away or marry mortals.[211] Thus they may be a reminiscence of old Earth goddesses.  But they are not exclusively female, since there are kings of the side, and as the name Fir side, “men of the side,” shows, while S. Patrick and his friends were taken for sid-folk.

The formation of the legend was also aided by the old cult of the gods on heights, some of them sepulchral mounds, and now occasionally sites of Christian churches.[212] The Irish god Cenn Cruaich and his Welsh equivalent Penn Cruc, whose name survives in Pennocrucium, have names meaning “chief or head of the mound."[213] Other mounds or hills had also a sacred character.  Hence gods worshipped at mounds, dwelling or revealing themselves there, still lingered in the haunted spots; they became fairies, or were associated with the dead buried in the mounds, as fairies also have been, or were themselves thought to have died and been buried there.  The haunting of the mounds by the old gods is seen in a prayer of S. Columba’s, who begs God to dispel “this host (i.e. the old gods) around the cairns that reigneth."[214] An early MS also tells how the Milesians allotted the underground part of Erin to the Tuatha Dea who now retired within the hills; in other words, they were gods of the hills worshipped by the Milesians on hills.[215] But, as we shall see, the gods dwelt elsewhere than in hills.[216]

Tumuli may already in pagan times have been pointed out as tombs of gods who died in myth or ritual, like the tombs of Zeus in Crete and of Osiris in Egypt.  Again, fairies, in some aspects, are ghosts of the dead, and haunt tumuli; hence, when gods became fairies they would do the same.  And once they were thought of as dead kings, any notable tumuli would be pointed out as theirs, since it is a law in folk-belief to associate tumuli or other structures not with the dead or with their builders, but with supernatural or mythical or even historical personages.  If side ever meant “ghosts,” it would be easy to call the dead gods by this name, and to connect them with the places of the dead.[217]