Fionn Mac Cumhaill and the worm of Lough Derg

Dublin Core


Fionn Mac Cumhaill and the worm of Lough Derg


Lough Derg--Fenian Cycle--Fion Mac Cumhaill--Red Lake


O'Donovan's account from a local of the origin of Lough Derg's name in a story from the Fenian Cycle


John O'Donovan, 1806-1861


Letter from John O'Donovan, Ballyshanny, 1st of November, 1835, p. 250,O’Donovan, John, Ordnance Survey Letters, Donegal: Letters Containing Information Relative to the Antiquities of the County of Donegal Collected during the Progress of the Ordnance Survey in 1835, p. 124


Four Masters Press, Dublin


1835 [2000]


Transcribed and edited by Michael Herity, MRIA


Citation for the purposes of criticism


Edited edition of letters


English with Irish text in Celtic script


Ordnance Survey Letters




54.616218, -7.876212

Text Item Type Metadata


"Finn Mac Cooil and his heroes were one day walking on the strand of this Lough, and Fin's notice was attracted by a large bone out of which a white little maggot was peeping, which when he observed, he put his thumb of knowledge in his mouth and squeezed it, and then stood for a short time over the bone in an attitude of surprise. Conan, the only coward in his service, came up to him and asked why he looked so seriously at the bone of an old horse. Fin replied, 'If this bone were thrown into the lake, that little maggot would grow the size of a monster and do much injury.' Conan, the Stolid took up the bone in his hand and addressing the little maggot said: 'Me-thinks thou hast not got the germs of a big beast, pity that thou shouldest not get enough to drink' with this, he flings the bone up into the air with all his might, and it fell almost in the middle of the Lough, which was then styled Lough Finn from the brightness of its waters. Cursed fool said Fin to Conan thou shalt yet pay dearly for that rash deed. The Devil may care said Conan. With that they passed away from the mountains in pursuit of Red Deer. In 12 months after they passed by the margin of the same Lough, an lo! they beheld with astonishment a multiform monster with three humps on its back resembling three round hills (cors), rising above the water making towards them with amazing rapidity and roaring most hideously. The alarmed Fingalians took flight leaving Conan a fearful distance behind. He was overtaken by the monaster and swallowed, arms and all! Conan felt the scorching heat of his stomach, and feeling that it was a very gloomy lodging he bethought him of a plan by which, if he could not effect his escape, he might at least be revenged of the Monster to whose strength he contributed so much. He took his meadog or side knife and cut thro' the monster's stomach and side which caused him to make for the shore again and vomit the soldier out. In passing through the lake he bled so profusely that it seemed all blood, and it continued so for a long time, which induced Fin to change the name of Loch Finn to Loch Derg."

Original Format

Hand written letters held in Royal Irish Academy



John O'Donovan, 1806-1861, “Fionn Mac Cumhaill and the worm of Lough Derg,” Digital Derg: A Deep Map, accessed April 21, 2024,