A supernatural smith who resided on the Isle of Man, "of so long-lived or mythic a nature as to be found living in all ages of pagan history." Cuillean — or Guillean — forged the magic sword, spear and shield for Concovar Mac Nessa, king of Ulster. The oracle of Cloghor directed Concovar to travel to the Isle of Man and commission the weapons from Cuillean, and the supernatural power they carried won him the sovereignty of Ulster.

Brash, writing in Antiquitates Manniae, argues that Cuillean may be identical with Manannán mac Lir. The points of identification are several: both are intimately connected with the Isle of Man, both are forgers of supernatural weapons, both are located in Ulster where Manannán is said to have reigned over the provincial fairy kingdom, and both fell into disrepute among the Christians. "Cuillean, too, fell into disrepute among the Christians, as did Mananan."

The Manx Gaelic phrase giolla Guillen — "the servant of Guillean" — survived into modern usage as a synonym for an imp of the devil, carrying the memory of the old smith long after the stories themselves had faded. Cuillean's cave was on Slieve Gullion in County Armagh, still remembered with a mixture of awe and unease in local tradition when O'Kearney recorded it for the Ossianic Society.