Slaine: The Roleplaying Game of Celtic Heroes

From the back of the core rulebook:

Kiss My Axe!
Immerse yourself in Celtic myth. Based on the legendary 2000AD comic strip saga created by Pat Mills and Angela Kincaid. Slaine is a brand new d20 role-playing game giving you background on Tir Nan Og, the Land of the Young in Celtic legend and full rules for playing the various characters found there. The innovative skill-based magic system lets all characters cast a spell or two if they wish, reflecting the superstitious beliefs of the Celts. Channel earth-power through your body, swelling to a monstrous size as you enter a warp-spasm. Learn the secrets of the feared gae bolga, a barbed and spiked spear which always causes fatal wounds, or hurl the dreaded tathlum, a concrete ball made from the brains of your dead enemies. Battle shoggey beasts, skull-swords and fomorian sea demons for the honour of your tribe. Enrage your enemies with the power of your poetic insults alone. Or throw subtlety aside and wield a flint great-axe, slaying fifty foes to the left of you and fifty to the right. You will not think it too many.

Inside you will find

Characters in Tir Nan Og
A complete guide to creating characters for use in the game - the mighty warriors and heroes of the Land of the Young, druids both wise and evil, cunning witches and wily thieves.

Feats
Over 40 new feats give you the chance to learn Salmon Leaps, Shield Growls and other Celtic battle techniques.

Combat
Full rules for naming weapons, taunts in battle, and warped warriors capable of taking on dozens of foes single-handedly.

Eyes Without Life, Sundered Heads, and Piles of Carcasses
Run mass battles of every size from a tribal cattle raid with just a hundred or so warriors on each side, right up to a full-scale Fomorian invasion of the Land of the Young.

Earth Power
Earth Power is the magic of Slaine's world, a subtle yet dangerous power derived from the land itself and frequently powered by human sacrifice.

Plus chapters on Tir Nan Og, goods and weapons, prestige classes, a complete bestiary, adventuring, gods and goddesses, and running campaigns in the Land of the Young.

Requires the use of Dungeons & Dragons Player's Handbook, Third Edition published by Wizards of the Coast