What is Mythology?
Mythology is not just a collection of tales "made up" by our predecessors. There is much truth in them.
Here are some links about myth, in general, and the role that they have played in civilizations since the dawn of language:
Definition of myth for a Mythology Class
The Illusion of Myth-less-ness
This collection of links is heavy on Gaelic lore, but if you look, there is lore from other Celtic nations, as well.
The Birth of Finn MacCumhail and the Founding of the Fenians
Folklore- Irish/Celtic Seasonal Celebrations
Irish Literature, Mythology, Folklore, and Drama--AWESOME collection of Mythology and Folklore, among other things
Medieval Sourcebook: The Destruction of Da Derga's Hostel, c. 1100
The Conquest of the Sons of Mil
Lebor Gabala Erenn--The Taking of Ireland, from The Irish Texts Society
WAR GODDESS: The Morrígan and her Germano-Celtic Counterparts, A dissertation by Angelique Gulermovich Epstein
Goddess Culture in Early Celtic Literature--A Thesis by Freb Hunt
The Second Battle of Mag Tured (Moytura)
Celtic Mythological Characters
Encyclopedia of the Celts
Gods and Goddesses of the Gaels-by Tina Deegan
Mannanan Mac Lir
Legends of Erin and Alba
An Irish Myth Concordance
The Fairy Faith in Celtic Countries by W.Y. Evans-Wentz--a WONDERFUL collection of folklore!!
CELT: Corpus of Electronic Texts
Tales of St. Patrick "Of the Bells" --From Lady Gregory