Origin of Irish nail-tailed monster story?

Supposedly there's a medieval Irish story from about A.D. 900 that tells of an encounter with a beast with iron nails coming out like spikes from its tail, and a horselike head. My source (a children's book) does not have a bibliography. Can anyone tell me any further details on this story? Is it a story of an Irish saint vs. a monter? Story of an Irish hero vs. a monster? A newspaper-type record? The story has given rise to an urban myth that stegosaurian dinosaurs roamed Ireland in medieval times, and I'm trying to track down the source of the urban myth.