The following are essential texts to read for seekers after the Atlantic 'Religion' (I'll try and get this in order and more complete at some point):

Irish Mythological tales:
Tain Bo Culainge, or "Cattle-Raid of Cooley": The battle-pageant of the Ulster Cycle, conducted from the sidelines by the fatal Morrigan. In a series of theatrical battle-scenes, often staged at river-crossings, the hero Cuchullain ('Whelp of the Cailleach') leads the action to recover the cosmic stud-bull of his leader. The allegory of rutting bulls and stags is a central theme, and a comment on inter-tribal rivalries and man's closeness to his natural origins in the ancient Atlantic Culture of Ireland. Its appendant tales also contain ideas of reincarnation – the Donn Cúailnge was supposed to have once been the magical swineherd of the fairy Bodb Derg, another possible literary figure in which Manannan existed.

Fled Bricremm, or "Feast of Bricriu": A central tale of the Ulster Cycle dealing with a competition between champions, again featuring Manannan who appears in the form of Curoi and a number of other foréms, providing a reminder that mankind's activities are ultimately guided by the destinies of the Otherworld.

Togail Bruden Da Derga, or "Siege of Da Derga's Hostel": A spectral odyssey of fate, visions and violence, important for its central character: The Cailleach. Although not named as such, she bestrides and directs the whole narrative, just as she does in the Tain in the guise of the Morrigan.

Imram Brann, or "The Voyage of Bran": The main Manannan text which (along with the accompanying Compert Mongan ("Conception of Mongan") provides a description of Manannan and his role as king of the Otherworld and his role in the lives of men and kings.

Tochmarc Étaíne, or "The Wooing of Etain": A tale of love between a fairy prince and a mortal woman, with transformations and reincarnations dy lioar! It is one of the most fabulous versions of the old Atlantic folktale of being abstracted into a fairy mound – sometimes an allegory for the strange 'sickness' of love which carries us off into another fevered world.

Lament of the Old Woman Berri – the other great Cailleach text, describing a fabulous ancient female recounting her youthful years as death approaches her. She is the ancestral sovereignty goddess who loved ancient kings, and she looks forward to the great flood of the Sea which is coming for her, no doubt to reunite her with her long-estranged consort, Manannan.

Online sources for historic Celtic literature:

University of Cork 'Collection of Electronic Texts' (CELT) – Excellent source for original texts and translations of texts concerning Ireland from the early medieval to the modern period.

Mary Jones 'Celtic Literature Collective' – Source for many translated works of Medieval Irish, Welsh, Latin literature.

16th-20thC Texts:

The Internet Archive – Very complete source for online works of 19th and 20thC scholarship. OCR often not edited. Lots of Kuno Meyer works.

Google Books – Scanned texts (OCR is also currently variable in quality). Want a 16thC incunabula? A Georgian newspaper? A rare 17thC polemical tract? Up to the end of the 19thC books are generally free, but be aware that access is sometimes limited. Here are a few recommended reads:

This article on Lithuanian Mythology.

tbc

Online Sources for Classics:

www.theoi.com – Amazing website examining all aspects of ancient Greek religion, with full citation of the textual sources on which they are based.

The Internet Classics Archive – Full text versions with translations of many of the most important works of ancient Europe and the Near East.

Perseus Digital Library – Online resource from Tufts University. Contains classical era texts as well as interpretative materials.

Online Celtic language resources:

Online version of the complete Dictionary of the Irish Language – Immensely useful resource, but not very user-friendly and searching can be difficult.

Recommended modern reads:

Anything by the 19thC antiquarian genius, Rev. Sabine Baring-Gould.

Anything by Karl Kerenyi or Mireja Gimbutas relating to European classical and paleo-religion. The works of Georges Dumézil  are seminal examinations of links between Indian and European cultures through those of the Near East.

Walter Burkert's excellent 'Greek Religion: Archaic and Classical'.

Anything by Professor John Koch at the University of Wales, Aberystwyth. He is a very significant contributor to the elucidation of Atlantic history and mythology ('Celtic Studies') and his output seems fairly prodigious, including an Encyclopedia of Celtic myth and folklore.

Professor Barry Cunliffe (Oxford University) is surely one of the other most respected and prolific academic researchers and authors in British archaeology, as well as producing more popular-styled books. His theories and commentaries upon the archaeological and historical evidence of the ancient cultures of Atlantic Europe have been a driving force behind the use of the biogeographical and cultural denominator 'Atlantic' in place of 'Celtic' in archaeological, historic and anthropological studies! Essential.

Any publications by the outstanding celtic numismologist R.D. Van Arsdell, author of the seminal book 'Celtic Coinage of Britain'.Some of his articles are available to read at: http://www.vanarsdellcelticcoinageofbritain.com

'Coins and Power in Late Iron Age Britain' by John Creighton (Cambridge University Press, 2009) – An excellent overview of Britain's Celtic Iron Age told through the story of its coins.

Books and publications by Miranda Aldhouse-Green, Professor of Archaeology at the University of Cardiff, who has a special interest in the European Iron Age and the Druids in particular. 'Caesar's Druids' (Yale, 2010) is a good introductory read.

The historian Professor Ronald Hutton also writes on the Druids, and although his primary works have encompassed the history of more modern interpretations of who and what the druids were, his interest in the history of paganism appears to be general. A master of the methodical (if sympathetic) demolition of an ahistoric argument!

Professor Jacqueline Borsje – another recommended academic 'heavyweight' – has written some very insightful papers on the medieval Irish world and its mythology. Her recent book is entitled: The Celtic Evil Eye and Related Mythological Motifs in Medieval Ireland.

The Festival of Lughnasa by the late Máire MacNeill – One of the great modern works of folklore and anthropology. Concerns the survival and contnued celebration of the ancient pagan festival in Ireland (also Britain and the Isle of Man). Essential.

'Elves in Anglo-Saxon England'(Pub. 2007 Boydell & Brewer) by Alaric Hall. Dense academic read, but worth it – a rising star in the field of medieval and early-modern Germanic literature who swerves into Gaelic and Latinate themes as well.

Israel Finkelstein's important 'The Bible Unearthed : Archaeology's New Vision of Ancient Israel and the Origin of Its Sacred Texts', New York: The Free Press, 2001. ISBN 0-684-86912-8. This deals with the archaeological evidence for the biblical providence of monotheistic Judaism which call the written traditions into question. In particular, it suggests that the kings Solomon and David as 'mighty kings'  were largely creations of literary monotheist Jahwism (of a period following the establishment of the Achaemenid Persian Empire) and that paganism persisted in the ancient kingdoms of Israel and Judah much later than the biblical narrative suggests.

TBC….