What makes Marie-Louise Sjoestedt’s take on a culture’s mythology different from other its type that I’ve read is that it is not a retelling of myths or a list of gods and what they were good at.  It is an exploration of the religious philosophy of Irish Celts.  Not so much about what they imagined as about how they imagined.  So, instead of looking for analogies between Roman or Norse gods and those of the Celts, the author attempts to show us the world through the eyes of the Celts, their place within it and the relationships between men, heroes, gods, and monsters.
There are a few specific examples, and a few myth cycles and epics discussed, as well a general myth-history of pre-Christian Ireland.  However, generally Sjoestedt looks at the close relationship between the natural and the supernatural.  The gods of the Celts were not conceptual archetypes living in some remote enclave.  They were tempestuous and multifaceted, and they lived in the stream, the forest, and the dark beyond the firelight.  Sex and violence, honor and strength, greed and sacrifice.  These were the things important to men and gods (and women and goddesses).  Sjoestedt discusses these with a clinical detachment, interested but non-judgmental.  A surprise for a book originally published in 1940, where some other books I’ve read have been quite prudish and aghast at cultural norms outside of contemporary Anglo-Christian.
The decision to focus on Irish Celts makes sense.  They were removed from continental Europe and thus less altered by Greek, Roman, and other cultures and religions.  Thus they likely had a more “pure” version of Celtic religious thought for much longer than their mainland European counterparts.
Myles Dillon’s translation is readable, and shows signs of a solid artist.  Thankfully missing is the sometimes deadly dryness and stilted language of the skilled translator/unskilled writer.  Though, on occasion the book does drag, wandering not so much off topic, but off point, it is a very readable work.  For anyone interested in the Celts and their religious ideas, this is a good starting point.  A foundation on which to hang further reading.  I prefer my history to be from the eyes of the person on the ground, and this look into the cultural identity of Celtic Ireland is a beginning.

You can follow me, if you so desire, on Twitter at @TheOmegaDork