Cé was the druid

Druid

A druid was a member of the priestly class in Britain, Ireland, and Gaul, and possibly other parts of Celtic western Europe, during the Iron Age....

 of Nuada of the Silver Hand. He was wounded in the Second Battle of Mag Tuired and fled southward till he came to Carn Corrslebe. Taking respite from his journey and his wounds aching, he saw ahead of him a beautiful plain full of flowers. He desired greatly to reach it, and when he did, he died. When his grave was made, a lake burst out over it, and the whole plain. It was given the name Lough Cé