The Fomorians from Irish mythology are steeped in mystery. It is unknown where they came from, or the length of time they had knowledge of Ireland, and what actually happened to them. Lady Gregory describes one Fomorian habitat as a glass tower in the sea. Many people describe them as sea fairies or sea monsters. My best guess is that they were Nephilim!
Historically, mapmaking was a fairly imprecise task . . .to fill blank areas on the maps, mapmakers used to include textual/graphic warnings of the dangers of going into uncharted territory. Such warnings took the form of sea serpents, dragons, mythical and, sometimes, even real creatures. The actual line "Here There Be Dragons" has been found only once, on the 16th-century Lenox Globe — but is too cool to give up.