Books

Folklore

The Táin, translated by Thomas Kinsella
Mythic Ireland by Michael Dames
Tales of the Elders of Ireland by Ann Dooley
Silva Gadelica by Standish Hogardy
Folklore of the Scottish Highlands by Anne Ross
The Silver Bough by F. Marian McNeill
Popular Tales of the West Highlands by J.F Campbell (available on Sacred-Texts)
Early Irish Myths and Sagas, translated by Jeffery Gantz
Ancient Irish Tales by T.P Cross and C. Slover
Fairy Legends and Traditions in Southern Ireland by Thomas Crofton Croker (available on Google Books)
Ancient Irish Poetry by Kuno Meyer
The Learned Tales of Medieval Ireland by Proinsias Mac Cana
A Golden Treasury of Irish Poetry by David Greene and Frank O’ Connor
Irish Folktales by Henry Glassie
The Folklore of the Isle of Man by Arthur William Moore (available on Google Books)
Myth and Magic: Scotland’s Ancient Beliefs and Sacred Places by Joyce Miller
Scottish Folktales and Legends by Barbara Wilson
Gods and Fighting Men by Lady Augustus Gregory (available on Sacred-Texts)

Folk Traditions

The Gaelic Otherworld by John Gregerson Campbell
The Year in Ireland by Kevin Danaher
Carmina Gadelica by Alexander Carmichael (Vol. 1-2 are available here & here, and Vol. 3 here)
A Handbook of the Scottish Gaelic World by Michael Newton
Irish Folk Ways by E. Estyn Evans
The Wisdom of the Outlaw by Joseph Nagy
Passing the Time in Ballymenone by Henry Glassie
Manx Calendar Customs by Cyril I. Paton
The Aran Islands by John M. Synge (available on Google Books)
Irish Bardic Poetry by Osborn Bergin
The Celtic Poets: Songs and Tales from Early Ireland and Wales by Patrick K. Ford
The Holy Wells of Ireland by Patrick Logan
Folk-lore of the Isle of Man by A.W Moore (available on Google Books)
Irish Spirit: Pagan, Celtic, Christian, Global by Patricia Monaghan
The Tory Islanders by Robin Fox
Irish Traditions by Kathleen Jo Ryan and Bernard Share, eds.
Ancient Legends, Mystic Charms, and Superstitions of Ireland by Lady Wilde (available on Sacred-Texts)

History and Society

Guide to Early Irish Law by Fergus Kelly
Dark Speech: The Performance of Law in Early Ireland by Robin Chapman Stacey
The Road to Judgement: From Custom to Court in Medieval Ireland and Wales by Robin Chapman Stacey
Pagan Celtic Ireland by Barry Raftery
The Story of the Irish Race: A Popular History of Ireland by Seamus MacManus
Picts by Annie Ritchie
Warlords and Holymen: Scotland AD 80-1000 by Alfred P Smyth
Scotland: A Concise History by Fitzroy MacLean
Ireland: A Concise History by Maire and Conor Cruise O’ Brian
Sex and Marriage in Ancient Ireland by Patrick C. Power
The Picts and Scots by Lloyd and Jenny Laing
The Age of the Picts by W.A Cummins
The Oxford History of Ireland by R.F Foster
Auraicept na n-Éces: The Scholars Primer, translated by George Calder
A Guide to Ogam by Damian MacManus
Early Ireland by Michael J. O’ Kelly
Pre-Christian Ireland: From the First Settlers to the Early Celts by Peter Harbison
The Festival of Lughnasa: A Study of the Celtic Festival of the Beginning of Harvest by Máire MacNeill
The Sheela-na-Gigs of Ireland and Britain: The Divine Hag of the Christian Celts by Jack Roberts and Joannce McMahon
Ireland Long Ago by Kevin Danaher
Cattle Lords and Clansmen by Nerys Patterson

Religion

The Sacred Isle: Belief and Religion in Pre-Christian Ireland by Dáithi Ó hÓgáin
The Secret Commonwealth and the Fairy Belief Complex by Brian Walsh
Survivals in Belief Among the Celts by George Henderson (available on Sacred-Texts)
Conversing with Angels and Ancient by Joseph Nagy
The Iona Community Worship Book by the Iona Community
Celtic Mythology by Proinsias Mac Cana
The Fairy Faith in Celtic Countries by WY Evans-Wentz (available on Sacred-Texts)
The Candle of Vision by George William Russell (available on Sacred-Texts)

Encyclopedias

The Lore of Ireland: An Encyclopedia of Myth, Legend and Romance by Dáithi Ó hÓgáin
A Dictionary of Celtic Mythology by James MacKillop
Myth, Legend, and Romance: An Encyclopedia of the Irish Tradition by Dáithi Ó hÓgáin (possibility that this is an earlier edition of The Lore of Ireland)

Gods

In Search of St. Brigid, Foundress of Kildare by Mary E. Pollard
Rekindling the Flame: A Pilgrimage in the Footsteps of Brighid of Kildare by Rita Minehan
The Festival of Brigit: Celtic Goddess and Holy Woman by Séamus Ó Catháin

Language

This section of the list has been moved to here.