The Farmer whose land it is on says the field has always been known as Gigmagog’s field, although has never known the identity of Gigmagog.
H
hashi
Apparently the Hag Calliagh Berra bewitched Finn MacCool. She persuaded him to dive into the lake to find her gold ring. When he emerged he had fallen under her evil spell, and had been turned into an old man with white hair. He eventually got her to remove the spell, although his hair remained white!