Looking for books on Irish/Celtic Mythology...

There are many but here are a few that are personal favorites or that I have used and know to be worthwhile.

Treasury of Irish Myth, Legend, and Folklore....William Butler Yeats (yes, the poet)

A Dictionary of Celtic Mythology

A Guide to Irish Mythology (Celtic Ireland) Maeve Walsh

Celtic Myths and Legends, Peter Berresford Ellis (Although many academics won't vouch for Ellis because his work is based on 19th century translations, it makes for interestng reading. Of course it depends on whether or not you are reading for pleasure, background for fiction or academic study)

Irish Myth and Legend, Lady Gregory

These would be excellent references - there are many more and more than a few websites devoted toward Irish myths.

I'd avoid Ellis; he can't read Irish or Welsh, and relies on nineteenth century translations. He's frequently ludicrously awful--at UCLA we explicitly tell undergraduates not to rely on Ellis. You might find these bibliographies helpful.

Just to get started, here's a link to a site that might help narrow an interest or inspire:

http://www.mythicalireland.com/mythology/tain/index.html

I read "Great Irish Tales of Fantasy and Myth" by Peter Haining and found it to be very informative.

I've found these two quite useful:
Myths and Legends of the Celtic Race by TW Rolleston. Being a Book Club edition, my copy doesn't have a publishing history, but the author died in 1920, so what he was collecting were probably late Victorian versions of Irish and Welsh myths.

The Fairy Faith in Celtic Countries by WY Evans Wentz has some theories that now seem bizarre, but which were quite mainstream when the author was writing and collecting the stories - the book was first published in 1911. His stories are more wide ranging too - he includes Cornwall, the Isle of Man and Brittany with Ireland, Scotland and Wales.

Evans-Wentz is still astonishingly useful, both as inspiration for fiction, and as a reflection even of current folk belief and custom.

Rolleston . . . meh. The problem with nineteenth century editions is two fold, not only that they were less than accurate as translators, but, more importantly, they're censored. They leave all sorts of stuff out, so I wouldn't even want to use them for fiction.

Even the compilation of Irish tales, Cross, Tom Peete and Clark Harris Slover. eds. Ancient Irish Tales, is better, in my opinion, than Rolleston. Rolleston wasn't collecting in the folkloric sense, but instead was producing retellings of the translations.

There's also a wealth of "current" folklore that's been collected from professional story tellers, and others, available from the Irish Folklore Commission, including a series of books and tapes of both English language and Irish versions.

I do not know if you can google him but try Michael Mullen. He is a member of the Irish Literature Hall of Fame-his writing is wonderful-and he is an expert.

Medievalist - that's interesting, about Rolleston. I hadn't realised (though I probably should have guessed) about censorship.