I recommend anything by Dáithí Ó hÓgáin. Consider, for example, his book, The Lore of Ireland: An Encyclopaedia of Myth, Legend and Romance (Cork: Boydell, 2006). His work lacks the layer of Celtic mysticism and romantic haze that often clouds the subject. Too often, modern enthusiasts - pursuing Neo-paganism, witchcraft/magic, or any number of other things - go to the subject of the early Irish texts to emphasize and sometimes distort, to serve their own purposes, so one needs to proceed with caution if one wishes to avoid the effects of those points of view.