This one is just bloody terrifying full stop. Monsters hiding in closets or under the bed are a thing of nightmares for most kids. A skeletal type monster with crazy hair would definitely terrify me as child.
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!