In a touristic brochure I find the following explanation of the name of Lough Derg: "There are several stories about the origin of the name ‘Lough Derg’ and they all are quite bloody! It is generally accepted that the name Lough Derg is an anglicised version of the Irish name Loch Deirgeirt meaning ‘Lake of the Red Eye’ or ‘The Red Lake of Deirc’. One folk tale illustrates the fear of satire that prevailed long ago among Irish kings and nobility. According to this story a celebrated poet from Ulster, Aithirne, was on a journey through Ireland. He made outrageous and unreasonable demands from every King through whose lands he travelled. These demands were met due to the fear of what the poet might say about the King on his travels, and to avoid provoking the Ulster King into war. On meeting the King of South Connacht, Eochaidh mac Luchta, at his fortress near the shores of Lough Derg, Aithirne demanded the King’s eye as a gift. The King had only one eye but he plucked it out at once and handed it to the troublesome poet. A servant led the blinded King to the shore of Lough Derg so that he could wash the wound. The servant remarked that the water was red from the King’s blood. The King replied that from that day on the Lough should be called Loch Derg Dheirc (lake of the red eye)."

I have the following questions:
1. Am I right that "Loch Derg Dheirc" is a mixture of an anglicization ("Loch Derg") and an Irish word in genitive, meaning "of the cavity" (i.c. the eye socket)?
2. If I'm right on 1, what would be the correct full Irish version?
3. On logainm.ie and on Wikipedia (and likewise in the brochure itself) I find that "Loch Deirgeirt" as the Irish name of Lough Derg. What kind of word is "deirgeirt"? Is it just related to "dearg"? If so, in what way? Or is the eye-thing somewhere hidden in this word? If so, where and how?

Thanks in advance.

I’d think:

dearg = red
dearc = eye, deirce = of eye (gen.)
deargdhearc = red eye
loch deargdheirce = lake of red eye

But obviously the last part isn’t that clear anymore, it has changed to -(dh)eirt, middle consonants became slender, so:
Loch Deargdheirce -> Loch Deirgdheirc -> Loch Deirgeirt

Thank you once again!

When you are faced with an Irish word for which the etymology is not clear... 6 times out of 10 it is because of the modern spelling rules and 4 times out of 10 it is because of ignorance and false information.

At least that is my experience hahaha

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