Celtic Who And What We Are
Thanks to conventions within the New Age movement (something I view with a shortened temper, if none of you hadn’t already guessed) and general pop culture conflation, the words Celt and Celtic have become mostly synonymous with Irish and Scottish culture and heritage. This is by no means inaccurate, but within the bounds of the pagan community, the “Celtic” traditions are by no means a simple term and isn’t used to describe a particular pantheon of deities like “Kemetic” or “Hellenic” refer to Egyptian or Ancient Greek.
“Celtic” will traditionally refer to different tribal groups that had relatively similar language and regional traditions during the Iron and Medieval age, though evidence of these cultures does date back into the Late Bronze Age, shoddy history and bad archeology have made these distinctions relatively difficult to make and so when someone uses the word “Celtic” in a pagan context they are usually referring to a number of different cultures. Technically speaking, Celts would refer to the tribal peoples that inhabited the British Isles, France, Germany, Italy and Poland, even significantly farther east. Many of these cultures were erased during the Roman expansions and even more so during the expansion of Christianity, so many theories are based of of the sister cultures.
The inhabitants of the British Isles, pre-Roman invasion were known as the Insular Celts because they all spoke a relatively similarly rooted language; they settled in Scotland, Ireland, the Isle of Man, Wales, Brittany and Cornwall, as well as mainland western Europe. Of these tribal groups, only six languages survive: Irish, Scottish and Manx Gaelic and Breton, Cornish and Welsh (though I’d be inclined to say that Breton and Cornish are somewhat behind in their modern revivals.)
Of the surviving mythologies, the most well documented are going to be the Welsh, Irish and Scottish; mostly through the virtue of the Christianization of the countries and the effort put in by Christian monks to document the history of the land, which inevitably put the oral legends and traditions into writing.
What is important to remember, at least from the perspective of being a hard polytheist (which means that every deity is a singular being) is that between the Welsh, Irish, Scottish, Brythonic and even Gaulish religions, there are going to be similar trends, stories, deities and even names; though we have to look objectively at the regional locations of the peoples and understand that they may have common root, but shouldn’t be conflated or equated to each other; as it’s highly likely that many of these cultures didn’t directly interact with each other on large enough scales to have complete religious reference like that.