The Invulnerable King

From the back of the book:

This campaign will take your players from Albion all over Tir Nan Og and beyond, as they quest to depose their tyrannical king, re-assemble an ancient stone circle, and save all the northern tribes from the combined might of the Fomorians and the Drune Lords.

All you need to play The Invulnerable King is this adventure, a copy of The Player's Handbook¸ and the Sláine RPG. This adventure is intended for use as the first of a four-part epic campaign, though it can easily be adapted to stand alone - either by the Games Master inventing his own continuation of it, or tying up the loose ends by having the Player Characters return the last of the Teeth to Osdann and sending them on a different mission for their King.