In Malory's Le Morte D'Arthur, Merlin says that the scabbard is worth more because as long as Arthur wore it, his wounds would not bleed. Such is also the case in the modern retelling, Mists of Avalon by Marion Zimmer Bradley in which Morgaine, or Morgan le Fey, fashioned the scabbard for Arthur.The one I like is more Arthurian legend then Celtic myth, and it is when the young King Arthur breaks his first sword in battle, the one he pulled out of the stone, and Merlin promptly leads him to the lake of Avalon where a lady of the lake presents the new king with an even better sword than his first -- that of Excalibur. As Arthur gazes upon the sword with wonder, Merlin warns him that the scabbard is worth more than the sword. When sheathed, the scabbard keeps the sword clean.
The only other sword I know of in Celtic mythology is the sword of the Daghda. What other ones are there? I have no idea how many swords there in that realm of myth.