Here’s the short answer: yes, the Celts do appear to have performed human sacrifice as part of their religious rituals. And, since the Druids were the religious/scholar/priestly social class, they almost certainly would have participated in human sacrifices, and probably officiated at them.
We have three sorts of data regarding Celtic human sacrifices. We have the words of Classical Greek and Roman writers, usually with a political agenda, and often reporting hearsay (Strabo for instance, was repeating the observations of the earlier no longer extant author Poseidonius), we have a few references in medieval Irish texts, primarily in the mythological tales, and we have archaeological data that is increasingly important.
First, here are some extracts about human sacrifice among the Celts from two Classical authors.
According to Strabo (64/63 B.C.E. – 21 C.E. at least) in his Geography (4.1.13):
The Romans put a stop both to these customs and to the ones connected with sacrifice and divination, as they were in conflict with our own ways: for example, they would strike a man who had been consecrated for sacrifice in the back with a sword, and make prophecies based on his death-spasms; and they would not sacrifice without the presence of the Druids. Other kinds of human sacrifices have been reported as well: some men they would shoot dead with arrows and impale in the temples; or they would construct a huge figure of straw and wood, and having thrown cattle and all manner of wild animals and humans into it, they would make a burnt offering of the whole thing (trans. by Benjamin Fortson, in Koch and Carey 1995, 18).
According to Julius Caesar (writing c. 15 March, 44 B. C. E.) De Bello Gallico 6.16):
All the people of Gaul are completely devoted to religion, and for this reason those who are greatly affected by diseases and in the dangers of battle either sacrifice human victims or vow to do so using the Druids as administrators to these sacrifices, since it is judged that unless for a man’s life a man’s life is given back, the will of the immortal gods cannot be placated. In public affairs they have instituted the same kind of sacrifice. Others have effigies of great size interwoven with twigs, the limbs of which are filled up with living people which are set on fire from below, and the people are deprived of life surrounded by flames. It is judged that the punishment of those who participated in theft or brigandage or other crimes are more pleasing to the immortal gods; but when the supplies of this kind fail, they even go so low as to inflict punishment on the innocent (trans. Anne Lea, in Koch and Carey 1995. 22).
Strabo’s reference to arrows is especially intriguing; there’s little or no archaeological data to support Celtic use of bows and arrows. Neither are mentioned in the medieval Irish tales, and the Irish words for bow and arrow are borrowed from Latin and Norse (Piggott 1975, 110).
The idea of a “wickerman” is reminiscent of references in both Irish legend and the second branch of the Welsh Mabinogi to men being inveigled into a specially built house, which is then set fire, immolating them. There is also a reference by Lucan, and the comments by later scholars as part of the Lucan scholia, in the Pharsalia,to three Celtic deities; Taranis said to have been propitiated by burning, Teutates by drowning, and Esus by hanging. Esus is mythologically similar to the Nordic deity Odin, also associated with hanging from a tree. And there is Tacitus’ account of the Roman attack on the Druid stronghold of Anglesey, which, although almost certainly politically motivated (Aren’t we Romans wonderful! We stopped those dreadful human-sacrifices by those nasty druids), he does refer to altars as “soaked with human blood” (I can’t help but wonder how he knew the blood was human). Boudicca also impaled victims during her rebellion in 60 A.D.
The best archaeological data supporting Celtic human sacrifice is the body of the man placed in Lindow bog in the first or second century C.E. We actually have the body (well, most of it) so well preserved that scientists were able to analyze his stomach contents to discover his last meal (a partially scorched grain cake). Lindow man was almost certainly a ritual sacrifice; he was strangled, hit on the head, and had his throat cut, in quick order, then surrendered to the bog. This pattern fits the “three-fold” death referred to in medieval Irish tales. What’s more, the man seems to have been of high social rank, and a willing victim. There are also other bog burials (the Tollund Man bog body in Denmark is very similar) in various places in Europe, as well as in grain storage pits and shafts in Britain, that, once they were no longer used for storage, had human bodies thrown in them, for instance at the Danebury hillfort. While Anne Ross in Pagan Celtic Britain is positive that the Danebury bodies were ritual sacrifices, most scholars are less certain.
A late Iron age shaft in Holzhausen in Bavaria with a post at the bottom was presumably used for impaling a human victim; the pole when analyzed had traces of human flesh and blood. In East Yorkshire, at Garton Slack a young man and a woman of about thirty were found huddled together in a shaft, a wooden stake between them pinning their arms together; the woman was apparently pregnant, since a fetal skeleton was found beneath her pelvis. Presumably the two adults were ritually killed for punitive purposes. There have also been several instances of foundation burials, often of children, which may or may not have been sacrifices (Green 1992, 183-84). Both bog and shaft burials seem particularly appropriate for cthonic otherworld-dwelling deities.
As for the evidence of Welsh and Irish tales about human sacrifice, the second branch of the Welsh Mabinogi tells of Efnisien jumping into the cauldron which brought the dead to life again, in an act of self-sacrifice which destroyed his life, and the cauldron. This myth is of course suggestive when one remembers the image on the Gundestrup Cauldron of a line of men moving towards what looks like a cauldron, with one man being submerged head first into the cauldron. The same image is also reminiscent of tales in which an over-excited Cu Chulainn is submerged into a sequence of vats to cool him off. Alternatively, one could argue that the image on the Gundestrup cauldron is of a victim (or perhaps a volunteer) being “donated” to a cthonic shaft.
Green, Miranda J. Dictionary of Celtic Myth and Legend. New York: Thames and Hudson, 1992.
Koch, John T. and John Carey eds. The Celtic Heroic Age. Malden, Massachustetts: Celtic Studies Publications, 1995.
Piggott, Stuart. The Druids. Frederick A. Praeger: New York, 1968. Reissued Thames and Hudson, London and New York, 1975.
Ross, Anne. Pagan Celtic Britain. Chicago: Chicago Adademy Publishers, 1996.
For more information on Lindow man and other bog bodies, see the bog bodies reading list.