I have really been reading a lot of games based on real world myths, and mostly Celtic myths. There are a lot of good ideas in these games, but none that felt perfect to me. I am still looking though!
Slaine the RPG of Celtic Heroes
Mongoose has released their 2002 OGL game, Slaine to PDF for what I think is the first time ever.
To begin with this is NOT a game of generic Celtic myths and heroes, this is a game for the 2000AD comic Slaine which borrows a lot from Celtic myth, but takes a number of liberties as well.
It also diverges from it's SRD/d20 3.0 (NOT 3.5) roots. So when reading, keep this in mind.
The book is very typical of a setting-type book.
We start with a number of classes. These have all be re-flavored to fit the mythos of the world better. So Tribal Warriors and Witches join the ranks of Druids and Thieves. Also we only have 3 races, Human, Dwarf and Warped-Ones (humans changed permanently by their interactions with the Beast Folk).
Next we come to skills and there are some differences here than the d20 norm.
We also get a new honor system. Enech: Honour and Reputation is used to tell the value of a warrior (his Sarhaed or Honor Price). It is also used when someone it wronged or challenged in a battle. In a lot ways it should be more important than XP. Tied to this are weirds (fate) and geas (taboos).
A strong collection of feats are presented. Including the fabled Warp Spasm and Salmon Leap.
Goods and Weapons is next and it deserves a careful read from the player. Afterall you might know that 3 gold piece is worth 3 cows, but that won't help you when all you have to barter for your new sword are chickens and pigs.
Combat is given special attention. In particular we get one on one combat, chariot combat and larger army combat. Useful for any d20 game in truth.
Magic and Spells are handled in a very different way. With each spell costing EP. Details are given about how gain and get EP for magical use.
We get some information on Slaine's world including the mythic version of the British Ilses (Albion, Alba, Cambria and Eriu or England, Scotland, Wales and Ireland respectively).
There is a section on adventures which includes some very interesting Prestige Classes.
We also get a run down on the Goddesses and Gods of the Tir Nan Og, and the bestiary of normal animals and more fantastic monsters.
Campaign ideas and notes.
All in all a good book if you are a fan of the comic or in Celtic myths in general. My only disappointments in this is some of the art is a low res scan and it looks very pixelated, the other is that there is no character sheet included. The character sheet for Slaine was one of the nicest ones from early in the d20 craze.
Bardic Lore: The Fachan
Celtic myth and lore is full of strange creatures. Some that don't quite have an analogue anywhere else. The Fachan is one such creature. Their might be similar creatures in other myths (I bet the Japanese or the myths of India have something like this) but none I can recall off the top of my head. This book gives us the background on the Fachan, 3.x style monster stats and some ideas to use it in your games. There is also a Fachan NPC and some notes on using the beastie as a character race. All in all not bad, and then when you consider the price then it is great.
Bardic Lore: Ogham
This is a well researched guide on Ogham, the written language of stones often seen near ancient Celtic settlements. This product blends historical findings with mythology to give us something very cool indeed. New ideas for Druids and Bards using Ogham are included along with a new feats, skill uses and revised spell lists. What is nice is the chart of the Ogham characters with sounds, English letter equivalents, and tree names. A lot of research went into all of this and the quality shows. Don't take it as a historical treatise on Ogham, but it is a great tool for a game. Nominally d20/3.5 but really the most of it can be used in any game.
Treasures of the Sidhe
Not a bad product. 45 new magic items of various degrees; most I thought were fine. It lacks art a lot of art, but for under 3.00 you are getting a lot of magic items and 1 new monster. Great if you are running a 3.x game bases around the Sidhe or the Seelie/Unseelie courts.