Supernatural stuff like this reported in medieval Irish sources is the focus of my current project, so naturally I'm interested in this sort of thing. After doing some research on both Google, and then in all the available medieval Irish annals (both the original Middle/Old Irish text and their English translation), I'm skeptical that such an annal entry even actually exists. It seems that this particular account is repeated in the exact same format on a variety of websites, whose common thread is their 'proof' that dinosaurs and humans coexisted, proving some Young Earth creationist theory.

Medieval Irish annals rarely report super weird phenomena like this which is what grabbed my attention; usually supernatural stuff can fall into categories like celestial portents (often directly referencing the Book of Revelations), battle-magic or encounters with humanoid supernatural entities. The only strange phenomena in or around the year 900AD that can be found in any of these sources is the beaching of a giant mermaid on a shoreline in Scotland. This particular entry does not fall into any category of supernatural phenomena I've ever encountered in medieval Irish sources, which makes me even more skeptical of its veracity: the supernatural in Irish sources comes from either Biblical (a la Book of Revelations) or folkloric inspirations, and as far as I know, a hissing beast with backwards iron tail isn't drawn from either tradition.

So, I've come to the conclusion that this entry was probably made up by someone years ago as evidence of a Young Earth creationist theory (after all, who would ever know where to find annals and other sources from the Irish medieval period that might disprove it?) and has consequently been parroted by similar websites and blogs, sort of like a really bizarre and misinformed version of broken telephone.