Wikipedia
Ceirean, Cirein-cròin or cionarain-crò was a large sea monster in Scottish Gaelic folklore. An old saying claims that it was so large that it fed on seven whales: Local folklores say this huge animal can disguise himself as a small, silver fish when fisherman come in contact with it. Other accounts state the reason for the disguise was to attract its next meal; when the fisherman would catch it in its small silver fish form, once aboard it changed back to the monster and ate him.
Gaelic
Translation
Notes
Seachd sgadain, sath bradain;
Seven herrings, a salmon's fill;
Seachd bradain, sath ròin;
Seven salmon, a seal's fill;
Seachd ròin, sath mial-mòr-mara
Seven seals, a large whale's fill
(Mial here is archaic; killer whales eat seals, but baleen whales do not.)
Seachd mial, sath Cirein-cròin
Seven whales, a cirein-cròin's fill
According to Forbes, "[In another saying] cionarain-cro here is substituted, as Avill be seen, for the cirein-croin in the former saving, and ranks second to the "great sea animal."
Forbes identifies the creature as a large sea serpent, but this is arguable. He also proposes it as a dinosaur -
"It is not known what this monster animal was, though it may well have been one of these "Giant fish-destroyers," so ably, inler-alia, described by Dr Carmichael M'Intosh, which waged war in sea and on land against all and sundry as well as against each other, viz., the gigantic Deinosaurs,[sic] some of which, notably the Atlantosaurus, reached to one hundred feet in length with a height of thirty feet, and proportionately awful of aspect."