Three later layers sit on top of the ancient material and are frequently mistaken for it.
The first is antiquarian. John Aubrey in the seventeenth century and William Stukeley in the eighteenth attributed Stonehenge and Avebury to the druids. They were wrong by a wide margin. The main phases of Stonehenge were built between roughly 3000 and 1500 BCE, over a thousand years before any druid is attested anywhere, and no ancient source connects druids with the monument at all. The association nonetheless survives in popular imagination and in modern ceremony held there.
The second is forgery. Iolo Morganwg, the Welsh poet Edward Williams, staged the first Gorsedd of Bards on Primrose Hill in 1792 and produced manuscripts purporting to preserve an unbroken Welsh druidic tradition. After his death much of that material was shown to be his own composition. Some of it, including the Gorsedd ceremonies now part of the National Eisteddfod of Wales, remains in living use and is valued on its own terms.
The third is deliberate reconstruction. Contemporary Druidry and Celtic Reconstructionist Paganism are separate projects. The first grew out of the eighteenth-century revival and is mostly not attempting historical accuracy; the second works from the Irish and Welsh texts and from archaeology and is. Neither claims descent from the ancient druids.
The pre-Christian religions themselves ended. What exists now is scholarship, folklore, and reconstruction by people who for the most part say so plainly.