Reddit reviews The Tain (Penguin Classics)

How the Irish Saved Civilization is a good read for a look at Ireland from roughly the fall of the Western Roman Empire through the so-called "Dark Ages".

The Tain (or Tain Bo Cualnge) is an ancient Irish myth about a battle that came about as a result of a cattle raid. It's talked about a fair amount in How the Irish Saved Civilization.

One side note: I've read How the Irish Saved Civilization, but I have not read the linked version of The Tain.

> I would like to find something with mythology of the gods and goddesses

If you would like a modern rendition of the Irish and Welsh tales, then Peter Berresford Ellis' Celtic Myths and Legends is a good choice.

If you are interested in delving into the lore directly, check out Táin Bó Cúailnge and other tales from Ireland's mythological cycles, or The Mabinogion for Welsh myths and legends. These are also available for free online, but I recommend buying the books if you can swing it because they tend to have higher quality translations and copious foot/endnotes that help put the stories in context and explain things to the modern reader.
> nothing goes into enough detail to connect dieties, or tell their stories, just general "this is the god of this, that and the other"

There are a lot of deities who we have only sparse information about. The majority of Celtic deities were local gods and goddesses, which gives us very few sources of detailed information. However, there are a good number of more widely-worshiped deities about which we know more. The myths and legends I mentioned above are the biggest sources of information about these figures, though we learn a lot of important info from the archaeological record as well.

You can check out http://www.maryjones.us/index.html which we link to in our sidebar. It has some decent articles and sources for many different Celtic figures and gods. If you would like a book that gives good info about archaeology and how it connects to the myths, check out Proinsias McCana's Celtic Mythology.

Wikipedia's Celtic and pagan projects have grown quite a bit, so you can even find some informative articles there with reference lists that can point you to more sources. For that, start with:

https://en.wikipedia.org/wiki/Celtic_deities

https://en.wikipedia.org/wiki/List_of_Celtic_deities