I’m going to focus on the second part of your question, about gods; I’d be very interested to hear from others on subjects such as material culture and legal traditions.

The relationship between pre-Christian deities in Ireland and Britain is deceptively complex, and views on the subject have changed significantly since the advent of academic Celtic Studies. In short, while early scholars were more apt to propose a unitary (Insular) Celtic pantheon, most modern researchers are much more skeptical. This is in large part due to a re-evaluation of the medieval texts on which much speculation about Irish and British pre-Christian beliefs has been based. While there are undoubtedly a few mythological figures known from both sides of the Irish Sea (and the middle of it), it has become harder to assert that these were necessarily deities worshipped throughout the region.

The evidence for the gods of pagan Britain and Ireland consists primarily of inscriptions and monuments, almost all from Roman Britain (England, Wales, and southern Scotland); and medieval narratives and poetry that purport to tell stories about the distant past. In that first category, you’ll notice that I specify Roman rather than pre-Roman. This is because pre-Roman Britain was not a literate society, its only writings consisting of a few coins from the decades immediately preceding the Roman conquest. These attest to a handful of regal names, but none indicate the names of deities. (Though some might depict them--for instance, the bearded entity crowned with a wheel and antlers on an early 1st century AD silver piece from Hampshire).

As for the medieval texts, the vast majority of these are from Ireland, with a few prominent examples from Wales. Crucially, these are all the work of Christians (often monks) writing centuries after conversion. Many centuries—the composers of masterpieces like the Táin Bó Cúailnge and the Mabinogi were about as far removed from the Christianization of their societies as we are from the Black Death. As such, they did not regard their colorful legendary characters as gods, or give any indication that they were ever worshipped as such. Rather, they depicted them as great kings, queens, warriors, artists, and magicians of the past; sometimes giant, or exceptionally long-lived, but never explicitly divine.

This creates significant problems for anyone trying to recover details of ancient pantheons from medieval stories. Generations of scholars have demonstrated how thickly enmeshed these stories are with the contexts of their composition, rather than their antiquarian settings. And not only with their local contexts; all the “mythological” texts clearly draw on the mainstream of medieval European Christian culture, from the Bible to Virgil to the international folkloric motifs known from tale collections like the Seven Sages.

Now, if the inscriptions created by actual pagan worshippers clearly referenced the same figures described long afterward in the medieval texts, it would be much easier to assert some degree of cultural continuity, historical memory, and/or oral tradition spanning these eras. But they don’t. With a tiny handful of exceptions (more on some of those in a moment), the gods we know were venerated in Roman Britain—or, for that matter, in other linguistically Celtic zones like Gaul or Celtiberia—do not appear in the medieval sagas of Ireland or Wales. Setting aside numerous dedications to the familiar Roman pantheon and other divine figures from throughout the Empire, we are left with indigenous gods like Coventina, Cocidius, and Belatucadrus. These seem generally to have been highly localized, sometimes associated with particular features of the landscape or kindred groups. Sulis, goddess of the salubrious spring water at Bath whom the Romans identified with Minerva, was worshipped almost exclusively at that site. This in turn should make us skeptical of a unified “Celtic” pantheon. A few British deities are mentioned by Roman historians, like the war goddess Andate/Andraste who appears in Dio Cassius’s account of Boudicca’s uprising. But these too are problematic--the Romans were not rigorous ethnographers, and Dio Cassius is particularly unreliable. And again, remember that in Ireland, we don’t even have these kinds of evidence--only a few ogam stones from the very late pagan period, which provide a few personal names (and crucial linguistic/philological information) but nothing on gods.

So, what about those exceptions I mentioned earlier? The marquee example is a name which appears as Nodens in a few Romano-British inscriptions, Núadu in several Irish texts, and Nudd in Welsh. These three forms are clearly related, though the exact Celtic root they derive from is a matter of some debate. The Romano-British Nodens is equated to the war god Mars in two dedications, one by a drill sergeant named Flavius Blandinus. Núadu Airgeadlámh (Nuadu Silver-Hand) appears as a warrior king in a few important works, most prominently Cath Maige Tuired (“The Battle of Moytura). But “Nudd” is not really a character in Welsh texts. Two figures, the obscure Edern and the somewhat less shadowy Gwyn, are identified as his sons (both are “ap Nudd”), but nothing is said of Nudd himself. Nothing in the medieval texts indicates any kind of divine purview for this figure, let alone a single cultic form shared between Britain and Ireland. And while Nodens was clearly worshipped as a god, his perceived nature has varied widely depending both on interpretations of the limited archaeological evidence and on the degree to which scholars attempt to make the medieval characters versions or manifestations of the same figure.

As it turns out, Nudd’s son Gwyn is another candidate for a shared British-Irish god. His cognate is the Irish Fionn mac Cumhaill (“Finn MacCool”), with gwyn and fionn both meaning “white, fair, holy(?)” from a Proto-Celtic *windos. While their patronyms are different, Fionn is sometimes described as a more distant descendant of Núadu, or another character with a similar name. Both characters are associated with hunting, and said to have dealings with otherworldly beings. But hunting was the universal pastime of medieval nobility, and sagas about the past foreground interactions with supernatural beings as a matter of course. Fionn is a heroic figure, a defender of Ireland against all manner of threats; Gwynn is a sinister being, who makes his literary debut by forcing a captive to eat his own father’s heart. By the later Middle Ages, he seems to have become a sort of “lord of the under/otherworld,” associated with nasty things like owls and bogs. There is no clear evidence for the worship of either character until the modern Neo-Pagan revival.

(cont.)