Reddit mentions of The Lore of Ireland: An Encyclopaedia of Myth, Legend and Romance

YES! I studied Irish mythology in University, so here a few of my favorite texts :)

The Tain is a lovely translation of the Tain Bo Cuailnge ^(sounds like "toy'n bo cool") which is the primary source of written mythology. It's dense.

Over Nine Waves is also, in my opinion, an imperative supplementary text on the myths and legends as well. It feels less academic.

The Lore of Ireland is just that - a book of Irish lore.

As for Samhain ^(I can't figure out how to properly give you a "sounds like" since I don't have little phoneme symbols - but basically like "sahwin") itself, I'm not sure whether holidays are out-right discussed in these texts. I will say, if you do find it - it'll be mostly in lore as opposed to myth or legend. Most Celtic holidays are focused on the changing of the seasons (like all holidays, really) and their connection to the Land of Eternal Youth (Tir na nOg - you can actually pronounce this one like you might expect it to be said) and the Tuatha De Danann ^(sub a "w" for the "th").

If you're looking specifically for myths dealing with faeries, they'll also be in lore. Myths/legends usually refer to the great heroes like Cuchulain ^("cuh-cul-lin") (there's a statue of him in the post office of downtown Dublin in honor of the Easter Uprising during the Irish revolution! Probably one of, if not the, most important myth/legend. In the war between gods and man, he almost single-handidly defeated Madb ^("mave") and her sons in a battle that last weeks/months/yeards depending the variation. He tied himself to a post as he was dying in order to look like he was still alive and held off attacks until crows landed on his shoulders and started eating his body. Basically. It's way better than my telling lol..) and gods and the cycles of power over the land itself.

Okay, fine, I'm done. Sorry for being so long-winded!

Oh! If you're looking for less heady material, I would also recommend Lady Gregory and Yeats. They were mythology nerds and wrote tons of plays/poems/retellings. L.Gregory's Grania is my favorite retelling of Grania and Diarmuid! I actually got a tattoo of one of the lines from the play in Ireland the first time I visited :)

Happy reading!

If you can get your hands on this, then I strongly recommend it. Albeit, it's worth noting that this deals with mythology (Gaelic, Celtic) from a distinctly Irish paradigm (most Celtic mythology is actually quite generic and can be seen repeating itself in different descended societies). Anyway, this was our bible whilst studying folklore here in Ireland. I'm not too sure about an online source, but I hope it helps nonetheless.

http://www.amazon.com/The-Lore-Ireland-Encyclopaedia-Romance/dp/1843832151