This is such a huge subject and the list is endless. I have an undergrad degree in Irish Studies and the book our class started with was "Early Irish Myths And Sagas" by Penguin Classics. It would be the most authoritative. My favourite for folklore is "Irish Folk Tales" edited by Henry Glassie, Pantheon Books, New York. It's easy to read, smooth-talkin. If that's not enough, go back in time to anything by The Four Masters or Geoffrey Keating. Add some Douglas Hyde. That's enough to insulate your spirit for the winter. And, too, John O Donohue's poetry often references the ancient myths and traditions and his Bird Of Paradise language will warm your heart. He's my top top favourite writer and poet. Slan.