Not. At least in the case of Norse paganism. I cannot speak to Celtic paganism, but seeing as how our sources and records for "Celtic paganism" are somewhere between slander at best and non-existent in most cases, I won't focus on it.
Borrowing from an older post of mine on Norse paganism as a whole...
Norse religious traditions are something that many of us in the western world are broadly familiar with, but only on the surface level. Odin is the All-father, Thor has a hammer, he fights giants, Loki is in there, and so on. However what we "know" about Norse mythology is mostly derived from a series of saga stories that were written down by Christians, and mostly one particular Christian (Snorri Sturluson) in Iceland centuries after conversion. The deities that we know and love, Heimdall, Tyr, Loki, all of whom are actually relatively unattested in archaeological evidence are common in the sagas, and vice versa, deities such as Ullr rarely appear in the saga literature despite far more evidence of a widespread cult based on place names. How are we to reconcile this difference between the literary evidence and the archaeological, especially in light of the reliability of the literary evidence compared to the archeological?
There are a few other written sources that are slightly more contemporary, such as the Poetic Edda (which predates the official conversion of most of the Norse world, but only just) and Saxo Grammaticus's Gesta Danorum which was written by a Danish Christian. Ibn Fadlan's account of the Rus people in his own journeys is also often used as a source on Scandinavia, despite the fact that he was writing about Russia and modern scholarship is increasingly nuancing the idea of Scandinavian domination of Russia.
To be clear though, using these sources to try and reconstruct the cosmology, theology, eschatology, beliefs, practices, rituals, and view point of Norse pagans is a fool's errand. The sagas have about as much to do with the practice of Norse paganism as Disney's Hercules does with Graeco-Roman paganism of the 4th century BC.
So with that out of the way what do we know about Norse paganism and what are our sources? (In the interest of time and space, I'm not going to be detailing each individual practice, ritual, and so on that we have evidence for, but rather detail a broad conclusions that some scholars have arrived at)
We are largely left with archaeological evidence (physical objects such as rune stones, artifacts, place name evidence, and so on), contemporary accounts from outside the Norse world, and extremely curated selections from the surviving corpus of Old Norse literature. So what do these sources tell us? What secrets can they reveal to the intrepid researchers of today?
In short, that the old Norse pagan religious tradition was elitist and extremely insular (not to mention barbaric, including human sacrifices and, if Ibn Fadlan is to be believed, the ritualized gang rape of slaves) with little popular participation and little buy in beyond the nobility. Norse paganism was hardly a core aspect of Norse "heritage" if the rapid and successful conversion to Christianity is a useful metric to go by. Indeed the religion likely varied extremely among the vast majority of the population and the paganism practiced in one part of Scandinavia likely bore little relation to that practiced in another. Evidence from across the Norse world shows that there was a great deal of variation in practices such as burial (cremation vs inhumation) and local cult popularity (as evidenced by the wide variety in theophoric place names).
The charismatic aspects of the religious tradition, veneration of Odin, ship burial/cremation, Valhalla, were probably the exclusive domain of the aristocratic elite of the Norse world. The average Norse person would not have been a participant in the same religious life as the elite of society. The average farmer, trader, slave, who lived in the Norse world almost certainly did not share the same conception of their own religious tradition as the elites of Norse society did. What good would Valhalla be to a farmer after all? Instead their worship likely focused around less well known deities with far less ostentatious displays of piety and worship.
Indeed it seems that the religion, such as it was, was incredibly tied to elite participation for legitimacy and practices. Elites in society, such as, but not limited to the King and his immediate family, were the ones who were keeping the religious practices going with ostentatious sacrifices including humans, horses, and other goods and food items and celebrated the deities and figures of the religion in their own oral traditions that would eventually be recorded by the same strata of elite members of society after conversion. They were also the ones who patronized the oral tradition of skaldic poetry that was eventually recorded by Snorri. Without elite buy in, the Norse pagan tradition could not, and eventually did not, maintain itself.
As Anders Andren says about the religion to sum up what I have covered:
Instead the religion must be regarded as a series of partly overlapping traditions, differing from place to place and from time to time, and also between different age groups, sexes, and social groups. Perhaps the shared Scandinavian features, such as boat graves and sacral place-names, should primarily be viewed as the religious expressions confined to an aristocracy with wide-ranging connections all over Scandinavia.