Meaning unknown, presumably of Gaelic origin. In Irish legend he was the young man who eloped with Deirdre, the beloved of Conchobhar the king of Ulster. Conchobhar eventually succeeded in having Naoise murdered, which caused Deirdre to die of grief.
From the name of the island off Scotland where Saint Columba founded a monastery. The name of the island is Old Norse in origin, and apparently derives simply from ey meaning “island”.