The Mysteries of Druidry by Brendan Myers - a great book that discusses Druidism from a specifically Irish perspective including both history and modern practice
Celtic Flame: An Insider's Guide to Irish Pagan Tradition by Aedh Rua - a greta look at one person's attempt to create a modern Irish pagan tradition. Useful for an Irish Druid on several levels, including ritual structure and thought provoking ideas on theology
The Apple Branch: A Path to Celtic Ritual by Alexei Kondratiev - not Irish specific but a must read fo rthe histrory of the different holidays; also full of important mythology and folklore
The Sacred Isle: Pre-Christian Religions in Ireland by Dáithí Ó hÓgáin - one of my personal favorites, a great look at the pagan Irish and Druids; the author does tend to look for a classical model for the Gods, so some of his ideas should be taken with a grain of salt, but overall very useful.
The Lore of Ireland: An Encyclopaedia of Myth, Legend and Romance by Dáithí Ó hÓgáin - essential quick reference for different Irish material, especially deities, heroes, places, and holidays
The Druid's Primer by L. Eastwood - again not Irish specific but an excellent look at the basic history of Druidism and a possible modern structure for it
Blood and Mistletoe: The History of the Druids in Britain by Ronald Hutton - an in depth look at the history of Druidism, particularly the revival period
The World of the Druids by Miranda J. Green - focused more on the history of Celtic religion and Druidism, including archaeological evidence
Druids, Gods & Heroes from Celtic Mythology (World Mythology Series) by Anne Ross - a basic introduction to Celtic mythology
Celtic Heritage by Rees & Rees - an indepth look at Celtic culture
A Druid's Herbal of Sacred Tree Medicine by Ellen Evert Hopman - lots of herbal lore, but also tidbits of Druidic history and lore about the holidays
Druidry and the Ancestors by Nimue Brown - not specifically Irish, but an excellent look at how to incorporate ancestor honoring into modern practice
War, Women, and Druids by Philip Freeman - a concise collection of ancient references relating to Celtic culture including Druids and bards
Druids Sourcebook edited by John Matthews - a collection of a variety of early and later references and articles about Druidism.
I'd also repeat my Irish recon reading list as it includes a lot of the important mythology:
Festival of Lughnasa by Maire McNeill - an in-depth look at the historic and modern celebration of Lughnasa, including a good deal of folklore and mythology
The Lebor Gabala Erenn - the story of the invasions of Ireland by the Gods and spirits and eventually humans.
Cath Maige Tuired - the story of the battle of the Tuatha de Danann with the Fomorians.
the Year in Ireland by K. Danaher - an overview of holidays and folk practices throughout the year.
The Silver Bough (all four volumes) by F. MacNeil - Scottish but extremely useful for understanding folk practices and beliefs
Fairy and Folktales of the Irish Peasantry by Yeats - a look at folklore and belief, important for including the Daoine maithe in your practice
Lady with a Mead Cup by Enright - useful look at ritual structure and society in both Celtic and Norse cultures
Celtic Gods and Heroes by Sjoestedt - discusses both the gods and tidbits of folklore and mythology
I'd suggest my own books as well, but that seems a bit self serving..