The Celtic deity clearly identified as the Celtic thunder god is Taranis (reconstructed 'Taranus). This name means 'thunderer' and is derived from a Celtic root word 'taran'.
The Celtic group of languages includes the insular Goidelic and Brythonic languages as well as the continental Gaulish, Lepontic and Celtiberian languages. In the Brythonic languages of Welsh and Breton 'taran' still means 'thunder' but the only surviving direct literary mention of Taranis is amongst the Gaulish gods. A first century c.e. poem by the Roman Lucan mentions the Celtic deities that Julius Caesar had found in Gaul. It describes the cult of Taranis in a scathing passage as 'those who appease with detestable blood the ferocious Teutates, the hideous Esus at his hearth and Taranis at altars no less inhuman than that of the Scythian Diana' (1). Lucan does not read as being the most dispassionate or disinterested of commentators.
The archaeological evidence, although there are only seven surviving inscriptions to him, can be found in Britain, the Rhineland, France and even Yugoslavia (this information is slightly old and I do not know whether that inscription is in Serbia, Croatia or Bosnia-Herzogovina). Some of these inscriptions link his name with Jupiter (Chester in England or Scardona in Yugoslavia) but he was also linked with the Roman god of wealth and the underworld - 'Dispater'. In considering the role of Taranis that such comparisons might bring to mind, it should be noted that Miranda Green has talked (2) of Taranis as a Celtic elemental entity before the Roman era and the comparison with Jupiter being made because of the thunder/lightning connection. Certainly the Roman chief god Jupiter was associated with a number of Celtic deities - both sky deities and mountain spirits. Though the comparison with Dispater may have meant that Taranis was viewed as having underworld (god of the dead) connections.
In the Berne Scholia, ninth century c.e. commentaries on Lucan, the author also compared Taranis to Dispater or Jupiter and says that people were sacrificed to Taranis by being burnt in a tub. Although the commentaries are talking of the types of sacrifice given to each of these 'major' Gaulish gods, it is still a correlation between the Celtic thunder god and the Norse/Germanic thunder god Thor/Thunor/Donar in that the Anglo-Saxon cemeteries have 'cremation' pots with the swastika on them indicating a dedication to Thunor.
However there is also a Celtic Hammer god: Sucellus. His name means 'the good striker'. Archaeological evidence suggests that Sucellus was especially popular in the Rhone and Saone valleys. Miranda Green, in discussing the hammer god (3), has said that the hammer 'may be a noisy symbol of thunder'. Certainly northern Europe's best known thunder god (the Norse Thor) was depicted with a sacred magical hammer (miollnir). Interestingly the length of the shaft of the hammer was significant to both Sucellus's hammer and to Thor's, although in diametrically opposed ways. The Norse myths tell of Loki distracting the dwarf making the hammer, so that the shaft ended rather shorter than usual, whereas the hammer of Sucellus comes at the end of a full length staff.
Certainly the representations of Sucellus have been felt to match those of Jupiter and the author Tertullian stated that Dispater had a hammer. So again, as for the known Celtic 'thunderer' Taranis, there is the comparison with Jupiter and Dispater. Although there may not be any immediately obvious link between the names 'Taranis' and 'Sucellus' but again looking at Thor, who was also called 'Asa-brag' (prince of the Æsir) and 'Hlorridi' (the loud rider), it becomes entirely possible that Taranis ('the thunderer') could also have been called Sucellus ('the good striker') by the Celtic tribes who knew him. Sucellus is also depicted with a cup or a small pot as well as the tall hammer. He is also often shown with a dog and/or as one of a divine pair with a goddess such as Nantosuelta.
In looking at the Celtic mythology that has come down to us we have 'The Mabinogion' for the Welsh or British tradition and the various Irish sources for the Gaelic tradition. The Mabinogion unfortunately, as it was compiled from eleventh century sources, contains 'a few faint memories of pagan deities and beliefs' (4) and there is only the faintest trace of the thunder god. Taran (no description) is described as the father of Glinyeu who was one of seven men who escaped when Evnissyen died in the cauldron and one of the seven men who were taken prisoner by Gwynn later in the book.
The surviving Irish mythology tells of the Tuatha De Danaan but there is no Taranis or Sucellus amongst them. Yet I believe that the important thunder god archetype is not missing. There is a god who '...used to work miracles for them, and to apportion storms and fruits...' (5). This god carries a club which is both an instrument of death (at the rough end) and an instrument of life (at the smooth end) in the same way that Thor's hammer was used both to kill (giants) and to bring back life (his goats on the journey to Utgard Loki). This Irish god was 'the good god' or the Dagda. Hilda Ellis Davidson has compared these gods and shown various points of similarity, such as the association with boundaries for both Dagda's club and for Thor's hammer, that both gods had voracious appetites, etc.
The Dagda was known to have a cauldron from which 'no company went away unsatisfied' which echoes the pot with which Sucellus was depicted and Dr. Daithi O hOgain said ' it is reasonable to assume that both the Gaulish 'Sucellos' and the Irish 'Dagdha' were pseudonyms for the same ancient Celtic deity' (6).
Although not under the elemental thunder name, this ancient deity was known throughout the Celtic lands. The early elemental deity growing into a benevolent if wild thunder god who had looked after his people, comparable to Thor.
1. T W Rolleston, The Illustrated Guide to Celtic Mythology, (Studio Editions 1993) page 8.
2. Miranda Green, The Gods of the Celts, (Alan Sutton 1986) page 67.
3. Miranda Green, Symbol & Image in Celtic Religious Art, (Routledge 1989) page 54.
4. Ronald Hutton, The Pagan Religions of the Ancient British Isles, (Blackwell 1991) page 47.
5. John T. Koch (ed) in collaboration with John Carey, The Celtic Heroic Age, (Celtic Studies Publications 1995).
6. Dr Daithi O hOgain, Myth, Legend & Romance: An Encyclopedia of the Irish Folk Tradition, (Ryan Publishing 1990).
H R Ellis Davidson, Myths & Symbols in Pagan Europe, (Syracuse 1988)
Miranda Green, Dictionary of Celtic Myth & Legend, (Thames and Hudson 1992)
Gareth King, Class Notes 1995. Snorri Sturluson (Anthony Faulkes trans) Edda (Everyman 1987)
The Mabinogion (Jeffrey Gantz trans) (Penguin Classics 1976)