Short answer: "Celtic mythology" is a label stretched over several cultures that never shared one pantheon (Irish, Welsh, Gaulish, Scottish), and in practice most of what people mean by it is Irish mythology plus a thick layer of 19th-century romanticism. There is a second honest caveat: the Gauls left almost no written myth at all, and the Irish and Welsh stories we do have were written down by Christian monks centuries after anyone last made an offering to these gods. What survives is a beautiful, edited, half-Christianised memory. Even so, it holds one of the strangest and best ideas in world mythology: an Otherworld you reach by walking into a hill, not by dying.
The gods of Ireland are the Tuatha De Danann, the people of the goddess Danu. They arrive out of the sky or the sea, beat the monstrous Fomorians, rule for a while, and are then beaten in turn by the Milesians, the ancestors of the Irish. Instead of leaving, they go down. Ireland is split in two: mortals take the surface, the Tuatha De take everything beneath it, the sidhe, the burial mounds scattered across the countryside. Over centuries the god-people shrank in the telling into the aos si, the good neighbours, the fairies. That is the actual origin of fairies. Not flower sprites. Defeated gods under the hill, still owed respect.
The main figures
| Name | Who they are | The story in one line |
|---|---|---|
| Tuatha De Danann | The god-people of Ireland | Lost the island to mortals and moved into the mounds, becoming the fairy folk |
| The Dagda | Father-figure and chief of the Tuatha De | Carries a club that kills with one end and revives with the other, and a cauldron that never empties |
| The Morrigan | War, fate and sovereignty | Appears as a crow over the battlefield, and seeing her means the outcome is already settled |
| Lugh | Master of every craft, called "of the long arm" | Talks his way into the court by being the only one good at everything, then kills the Fomorian king Balor, his own grandfather |
| Brigid | Poetry, healing, smithcraft, fire | The deity who survived the conversion and came out the other side as Saint Brigid, feast day included |
| Cu Chulainn | The hero of Ulster | Holds the province alone in a battle frenzy that twists his body out of shape, then ties himself to a standing stone so he will die upright |
| Fionn mac Cumhaill | Leader of the Fianna warband | Burns his thumb cooking the salmon of knowledge, sucks it, and takes wisdom meant for another man |
| Rhiannon | Welsh, sovereignty and horses | Rides a white horse nobody can catch, is falsely accused of killing her son, and carries the punishment anyway |
| Arawn | Welsh, king of Annwn, the Otherworld | Trades shapes and lives with a mortal prince for a year, in a story about honour rather than damnation |
Where the stories come from
| Source | What is in it | Written down |
|---|---|---|
| The Mythological Cycle | The Tuatha De Danann, the invasions of Ireland, the battles of Mag Tuired | Manuscripts from the 11th century onward |
| The Ulster Cycle | Cu Chulainn, Queen Medb, the cattle raid of the Tain Bo Cuailnge | Copied in the 12th century from older, now lost versions |
| The Fenian Cycle | Fionn, Oisin and the Fianna, hunting and journeys to the Otherworld | Popular from the 12th century, still being sung in the 19th |
| The Mabinogi (Welsh) | Pwyll, Rhiannon, Branwen, Arawn and Annwn | Two manuscripts, roughly 1350 and 1400 |
Those dates are the honest part. Every one of these texts was written by Christian scribes in monasteries, centuries after the last person worshipped the Dagda. They kept the stories because they were good stories and part of the national past, and they edited as they copied: gods get demoted to kings and ancestors, the odd biblical reference appears, the pagan machinery left running but unlabelled. Reading Celtic myth means reading a copy made by people who did not believe it.
The Otherworld, and the night that became Halloween
The most distinctive idea in the whole tradition is that the Otherworld is not the afterlife. Tir na nOg, Annwn, Mag Mell: these are places, next door and geographically real. You get there through a mound, across a lake, west by boat, or by walking into a mist that was not there a minute ago. No judgement, no punishment, just different rules: no illness, no ageing, and time running at its own speed, so three days there can cost you three hundred at home. Compare a Norse warrior carried off by valkyries or a Greek soul crossing a river. Here nobody has to die at all.
The nights when that boundary thins are the festivals. Samhain, at the start of November, closes the harvest and opens the winter, and on it the mounds stand open: bonfires, the dead visiting, disguises so the visitors would not know you. Christianity later set All Saints and All Souls beside it, and Irish and Scottish emigrants carried the customs to America in the 19th century, where the turnip lantern became a pumpkin and going house to house became trick-or-treat.
Quick questions
Who are the main Celtic gods?
In the Irish material: the Dagda, the Morrigan, Lugh, Brigid, Danu, Nuada of the silver arm, and Manannan mac Lir of the sea. In Wales: Rhiannon, Arawn, Arianrhod, Gwydion. In Gaul, Roman writers hand us names like Cernunnos and Taranis and almost no stories to go with them. There is no single Celtic pantheon the way there is a Greek or a Norse one, and any tidy list is smoothing over four cultures.
Is Halloween really Celtic?
Partly, and the honest answer is layered. The root is Samhain, a genuine pre-Christian Irish festival of the year's turning. On top of it sits the medieval Christian calendar of All Saints and All Souls, which supplied the name (All Hallows' Eve). On top of that sits American reinvention in the 19th and 20th centuries: pumpkins, costumes as an industry, trick-or-treat. Halloween descends from Samhain. It is not Samhain.
Are Celtic and Norse mythology related?
They are separate traditions with a distant shared ancestor. Both descend from Indo-European roots, which is why you find recurring shapes: a thunder god, a sacred cauldron or drink, symbols of the same old ideas. Later they met for real, when Norse settlers founded Dublin in the 9th century and lived alongside the Irish for two hundred years, leaving borrowings at the edges of both literatures. But Odin is not a Celtic god, the Dagda is not a Norse one, and the triple spiral you see on Instagram is neolithic, older than either.
Celtic myth is not in MythologyNow yet, and I would rather say so than pretend. What is there today is Greek, Norse and Egyptian, narrated as short episodes: the gods and the myths. The Tuatha De are on the list.
