There is a wealth of information on Celtic Mythology on the internet.  One excellent resource is the Celtic Myth Podshow where you can listen to retelling of various stories.

Celtic Myth Podshow Bringing the tales and stories of the ancient Celts to your fireside.

Some other websites that may be of interest are listed here.  For further reading, we suggest you check out the Fishpond.com.au recommendations shown below.

Irish Literature, Mythology, Folklore & Drama
Sacred Fire - Celtic Mythology
Mythography - Celtic Myth & Art