Manannan knew their relationship was doomed because Cuchulainn was mortal and Fand was a fairy; Cuchulainn's presence would destroy the fairies. Manannan then erased both Fand's and Cuchulainn's memory of each other.
Fand was a minor sea goddess in Celtic mythology, probably originating from an ancient Manx deity. Along with her sister, Liban, she was a goddess of pleasure and health.