I can't recommend this wonderful book highly enough. Myths and Folk Tales of Ireland contains stories collected in the West of Ireland by Jeremiah Curtin during the end of the nineteenth century. Jeremiah Curtin was an Irish American ethnographer, working for the Smithsonian Institution. He did not speak Gaelic himself, but he hired Gaelic-speaking interpreters to record traditional stories from the oral tradition, and then translate them into English. Because of Mr. Curtin's faithfulness to the original sources, the stories are written in a wonderful prose, full of poetic, traditional phrases. We can hear the voices of nineteenth century Gaelic-speaking storytellers speaking from the page.
The stories in this book fall into two groups: Irish versions of widespread folktales such as "Cinderella", "The magician and his pupil," or "The giant with no heart in his body", and native Irish Fenian tales, about Finn MacCool and his companions. Reading them leads you into another world, where people would gather in the evening, by the light of a peat fire, and listen to a storyteller speak about heroes and lucky younger sons, giants, magicians and monsters. As an amateur storyteller, I have found this book to be a great resource, specially for Saint Patrick's Day, but suitable for all occasions. The stories practically tell themselves. I have found "The fisherman's son and the gruagach of tricks" to be specially popular, maybe because of the thrilling chase at the end.
I can also highly recommend another book by Jeremiah Curtin: Irish Tales of the Fairies and the Ghost World. This a collection of more homely folk-tales, full of great Halloween storytelling material.