What is Celtic, exactly? This is a surprisingly difficult question to answer. The term has been used in many different ways by both scholars and laypersons alike. Do we define it based on a language group? A geographic area? Common elements of an art style? Not even scholars who study the field agree on precisely what it means, but what we do know is that "the Celts" as some sort of common, unified people wasn't really a thing historically - they were distinct groups and tribes that had their own unique traditions.
- This is important to remember when researching "Celtic myths" because you will often have better luck looking for stories within specific traditions, such as Welsh lore or Irish lore.
Yes, this is very true. I'd be interested in both Irish and Welsh. As someone who is linguistically inclined, I divide by language group; so, as Irish comes from Goidelic and Welsh comes from Common Brittonic that would be a starting point for me to distinguish them. We also know that many of the so-called Celtic worship sites in the British Isles were created by pre-Celtic peoples and thus repurposed - looking for any original meaning in these structures thus seems pointless.
Weren't Celtic peoples pre-literate? Yes. Yes they were; indigenous Paganisms of Europe were all oral cultures. The implications this has are numerous, but most relevant here is to raise the question - how is it that we have any of these ancient tales today anyway? Who was it wrote them down (because it wasn't the people who told the stories)? Learning about the context through which these stories were passed down to us is important to understanding their context.
- Celtic lore in particular was subjected to some whimsical and creative scholarship in the early days of anthropology. That's a polite way of saying that scholars basically made stuff up. Modern Druidry itself draws inspiration from the spurious scholarship of Edward Williams (better known as Iolo Morganwg). Even today it can be difficult to parse what was fabricated from what was genuine.
Indeed, one of the differentiating factors between North-Central European and Mediterranean religious cultures is that the latter favoured more writing, so we can 'reconstruct' Classical Mediterranean beliefs/practices better than North-Central European ones. This has led to a lot of mythmaking, especially by Early Modern and Victorian romanticists. The idea of 'fairies' is a god example of this, I think. It also goes with the Noble Savage idea. I don't see this as being as much of a problem as it may otherwise be though, as 'Paganism' (however ill-defined) is not book-led and can thus change much easier according to time and place. I see no issue with such change.
Is this Celtic stuff a fad? As if these two issues didn't complicate things enough, all things Celtic has enjoyed popularity today and modern people enjoy creative license with what material is available. Is this a bad thing? A good thing? It depends on your point of view and what you are looking for. The perspective of the Druid order I belong to tends to respect the flow of creative inspiration while also keeping us mindful of sourcing.
- What this means is that the question of whether or not something is "authentic" to some constructed standard is less relevant than if it authentically inspires joy and happiness in your life. Most of us aren't scholars - we don't need to be concerned with pedantry and living religion evolves and changes over time.
As a Brit, I do dislike it when our culture is appropriated by people who don't really know what they're doing with it and start talking nonsense history. I must admit that is annoying. If they want to be Celtic Pagans, go ahead, power to you, but please don't pretend you have historical accuracy when you don't. Same goes for the Bretons, I imagine. I accept intuition, innovation etc., but history=/= religion.