The Celtic Dragon Myth, by J.F. Campbell, , at sacred-texts.com
394. So the fisher's three sons, who went upon three ways by courage, cunning, and patience, by using gifts earned from thrice three talking creatures of earth and air, had conquered nine foes. They had overcome three giants with many heads, and a giantess, owners of three metallic castles; a dragon with three heads who came from the sea; a mermaid whose life was in an egg which was won from the sea; the great Turk, his army, and his fleet, which brought him from the sea; a lion, in three hard fights, who wished for water and therefore belonged to the sea; a giant who could only be slain with an egg which came out of the lion. Each had won a princess and was heir to a kingdom; one had got three metallic castles besides, and they had overcome six fortresses. Now they have to encounter the weakest, and the least, and the most dangerous of
their foes in the seventh castle. And this is the way in which the story was told in the Isles.