Translation according to P. W. Joyce:
Derry; Irish doire, an oak grove, from dair, an oak. Sometimes it is applied to any grove, as in Derryoghill. It was anciently a neuter, of which traces still remain (p. 10) [reproduced below].
Neuter Gender and Neuter Eclipsis. In Old Irish there was a neuter gender, which has dropped out, for it does not appear in Middle or Modern Irish: we have now only two genders, masculine and feminine. An old Irish neuter noun caused an eclipsis, or what was equivalent to an eclipsis, in the word immediately following. For example, the plain of North Dublin is called Magh-Breagh in Modern Irish; but in Old Irish it is Mag-mBreg, where the B is eclipsed by m; for Mag (now Magh) is a neuter noun. Under the same influence, if the word following a neuter noun begins with a vowel, the letter n is prefixed to the vowel. But although the neuter gender has passed away, its effects are to be seen, even in modern anglicised Irish names, just as the foot-prints of prehistoric animals are now often found, after thousands of years, on the surface of hard rocks. Many of the old eclipsing letters inserted by the neuter nouns still remain and cause eclipsis in our present names. Take these two examples, one with consonantal neuter eclipsis, the other with n before a vowel. Dungrud is still the name of a place in the Glen of Aherlow at the foot of the Galty Mountains, taking its name from Slieve-Grud (Sliabh-gCrot), which is the old name of the Galtys themselves. Here the root word is Crot (meaning in gen. plural, "harps," according to the old legend, for which see my "Wonders of Ireland," p. 106). But as both Dun and Sliab are neuter, Crot is eclipsed to grot or grud (gCrot) when following either, an eclipsis which still remains in "Dungrud," now in everyone's mouth in the neighbourhood, where the g of "Dungrud" corresponds to the track of the animal in the rock. For a vowel initial eclipsis, Lough Neagh is a good example. The root word is Each, representing Echach, gen. of Echo (the name of the man who gave name to the lake, according to the legend in the "Book of the Dun Cow"). But as Loch is a neuter noun, we have "Lough n-Eagh" or Lough Neagh instead of Lough Eagh. As a matter of fact, some early Anglo-Irish writers call it "Lough-Eaugh," dropping the N, as we often neglect both eclipsis and aspiration in our present anglicised names (See p. 4, XI.) Observe, though the origin given here for these two place names (Dungrud and Lough Neagh) is legend, the eclipsis is not legend, but actual grammatical fact, and quite correct. All this neuter eclipsis has been well explained from Zeuss, by the Rev. Dr. Hogan, S. J., in his "Battle of Rosnaree," and I have taken full advantage of his explanation here. Another excellent example of consonantal neuter eclipsis is the common word da-dtrian, two-thirds (da two, trian a third), where da is, or was, neuter, and eclipsed the t; and there the eclipsing d remains to this day. In by far the greatest number of cases this neuter eclipsis with its eclipsing letter has in the course of long ages, disappeared with the dispappearance of the old law itself; just as the stones of ancient buildings drop out when the mortar that held them together is gone. But in a few instances they still remain, as in the examples given above, and in others which the reader will see as we go along; for I will often direct attention to them. This phonetic law is not explained in my first two volumes; for the good reason that I did not know it myself when I wrote them. The existence of a neuter gender, with its effects, is a late discovery by the great German Celtic scholar Zeuss, and is fully set forth in his immortal work "Grammatica Celtica." Several instances of this neuter eclipsis occur in the first two volumes of this work, though they are there let pass unnoticed, such as Moynalty, in Dublin and Meath (vol. i. p. 424). I remember well about Moynalty. The two words of which it is composed are Magh or Moy, a plain, and ealta, bird-flocks (gen. plural). But when these two words were compounded in the old records, the letter n was inserted - Magh-n-ealta, plain of the bird-flocks. What brought the n there? This was a sore puzzle to me: and no wonder; for O'Donovan failed to explain it. And then the gratifying surprise when Zeuss's discovery cleared up the whole mystery and many another like it. Some of those neuter nouns, instead of eclipsing as of old, now often cause aspiration, where, according to modern rules there should be no aspiration, which I think is a remnant - a mere weakening - of the old eclipsing influence. I will direct attention to some cases of this kind as we go along.