Excalibur:
from the Latin Caliburn, used primarily by Geoffrey of Monmouth, who derived it from the Welsh name Caladfwlch, which in turn comes from Cuchulainn's sword Caladbolg, meaning "Hard Lightning."
The sword of King Arthur, forged on Avalon (according to Geoffrey), and given to him by the Lady of the Lake to replace the Sword in the Stone (according to Robert de Boron and subsequent writers). It had the magical property of being able to cut through metal and never breaking; but better yet, the shieth made the wearer unable to be injured in battle. When Merlin asked the young Arthur which was better, the sword or the shieth, Arthur answered the sword. Merlin said that this decision would cost the king his life. Morgan le Fay would later steal the scabbard and switch it with a fake one, thus enabling Arthur's mortal wound at Mordred's hand. Excalibur is then said to have been thrown into the lake where it came from, presumably the lake surrounding the Isle of Avalon.
The Irish origin for the name is no mystery; Irish communities settled along the southern coast of Wales from the fifth century onwards; even without this, there was certainly communication and trade between Ireland and Wales. Also, several Irish figures show up in the story "Culhwch and Olwen":
Crychwr the son of Nes, and Cubert the son of Daere, and Percos the son of Poch, and Lluber Beuthach, and Corvil Bervach
Conchobor mac Nessa; Cu Roi mac Daire; Fergus mac Roigh?; Laegaire Buddach; and Conall Kernach.