Translation according to P. W. Joyce:
Derravonniff in Galway; Doire-dha-bhanbh [Der-aw-vonniff], oak wood of the two bonnivs or sucking-pigs. For two objects in names, see vol. i. p. 247 [reproduced below].
I never saw it stated that the number two was in Ireland considered more remarkable than any other; but from whatever cause it may have arisen, certain it is that there existed in the minds of the Irish people a distinctly marked predilection to designate persons or places, where circumstances permitted it, by epithets expressive of the idea of duality, the epithet being founded on some circumstance connected with the object named; and such circumstances were often seized upon to form a name in preference to others equally or more conspicuous. We have, of course, as they have in all countries, names with combinations of other numbers, and those containing the number three are very numerous; but the number two is met with many times more frequently than all the others put together. The Irish word for two that occurs in names is dá or dhá, both forms being used; dá is pronounced daw; but in the other form, dh, which has a peculiar and rather faint guttural sound, is altogether suppressed in modern names; the word dhá being generally represented by the vowel a, while in many cases modern contraction has obliterated every trace of a representatibe letter. It is necessary to bear in mind that dá or dhá generally causes aspiration, and in a few cases eclipses consonants and prefixes n to vowels (see pp. 19 and 21, supra). We find names involving the number two recorded in Irish history, from the most ancient authorities down to the MSS. of the seventeenth century, and they occur in proportion quite as numerously as at the present day; showing that this curious tendency is not of modern origin, but that it has descended, silent and unnoticed, from ages of the most remote antiquity. There is a village and parish in the north-west of Tipperary, on the shore of Lough Derg, now called Terryglass: its Irish name, as used in many Irish authorities, is Tir-da-ghlas, the territory of the two streams; and the identity of this with the modern Terryglass is placed beyond all doubt by a passage in the "Life of St. Fintan of Clonenagh", which describes Tir-da-glas as "in the territory of Munster, near the River Shannon". The great antiquity of this name is proved by the fact that it is mentioned by Adamnan in his "Life of St. Columba" (Lib. Ii., Cap. XXXVI.), written in the end of the seventh century; but according to his usual custom, instead of the Irish name, he gives the Latin equivalent: in the heading of the chapter it is called Ager duorum rivorum, and in the text Rus duum riculorum, either of which is a correct translation of Tir-da-ghlas. There is a subdivision of the townland of Clogher in the parish of Kilnoe, Clare, called Terryglass, which has the same Irish form and meaning as the other. In the Book of Leinster there is a short poem, ascribed to Finn Mac Cumhail, accounting for the name of Magh-da-ghéisí, in Leinster, the plain of the two swans; and the Dinnsenchus gives a legend about the name of the river Owendalulagh, which rises on the slope of Slieve Aughty, and flows into Lough Cooter near Gort in Galway. This legend states, that when Echghe [Ekte] a Dedannan lady, married Fergus Lusca, cup-bearer to the king of Connaught, she brought with her two cows, remarkable for their milk-bearing fruitfulness, which were put to graze on the banks of this stream; and from this cricumstance it was called Abhainn-da-loilgheach, the river of the two milch cows. According to the same authority, Slieve Aughty took its name from this lady - Sliabh-Echtghe, Echtghe's mountain. Several other instances of names of this class, mentioned in ancient authorities, will be cited as I proceed. This word loilgheach appears in the name of a lake in the north of Armagh, near the south-west corner of Lough Neagh, called Derrylileagh, which means the derry or oak-grove of the milch cows. Though this peculiarity is not so common in personal as in local names, yet the number of persons mentioned in Irish writings whose names involve the number two, is sufficiently large to be very remarkable. The greater number of these names appear to be agnomina, which described certain peculiarities of the individuals, and which were imposed for the sake of distinction, after a fashion prevalent among most nations before the institution of surnames. (See Vol. II., Ch. IX.). One of the three Collas who conquered Ulster in the fourth century (see p. 137) was called Colla-da-Chrich, Colla of the two territories. Da-chrich was a favourable sobriquet and no doubt, in case of each individual, it records the fact of his connection, either by possession or residence, with two countries or districts; in case of Colla, it most probably refers to two territories in Ireland and Scotland, in the latter of which he lived some years in a state of banishment before his invasion of Ulster. In the Martyrology of Donegal there are nine different persons mentioned, called Fer-da-chrich, the man of the two territories. The word Dubh applied to a dark-visaged person is often followed by da; thus the Four Masters mention two persons named Dubh-da-bharc, the black (man) of the two ships; four, named Dubh-da-chrich; eight, Dubh-da-bhoireann (of the two stony districts?); two, Dubh-da-inbher, of the two estuaries; one, Dubh-da-ingean, of the two daughters; four, Dubh-da-leithe, of the two sides or parties; and two, Dubh-da-mhagh, of the two plains; and in the Martyrology of Donegal Dubh-da-locha, of the two lakes. Fiacha Muilleathan, king of Munster in the third century, was called Fer-da-liach, the man of the two sorrows, because his mother died and his father was killed in the battle of Magh Mucruimhe on the day of his birth. The father of Máine Mor, the ancestor of the Hy Many, was Eochaidh, surnamed Fer-da-ghiall, the man of the two hostages. Many more names might be cited, if it were necessary to extend this list; and while the number two is so common, we meet with few names involving any other number except three. It is very natural that a place should be named from two prominent objects forming part of it, or in connection with it, and names of this kind are occasionally met with in most countries. The fact that they occur in Ireland would not be considered remarkable, were it not for these two circumstances - first, they are, beyond all comparison, more numerous than could be reasonably expected; and secondly, the word dá is usually expressed, and forms part of the names. Great numbers of places are scattered here and there through the country whose names express position between two physical features, such as rivers, mountains, lakes, etc., those between two rivers being the most numerous. Killederdaowen in the parish of Duniry, Galway, is called in Irish, Coill-eder-da-abhainn, the wood between two rivers; and Killadrown, in the parish of Drumcullen, King's County, is evidently the same word shortened by local corruption. Dromderaown in Cork, and Dromdiraowen in Kerry, are both modern forms of Druim-'dir-dhá-abhainn, the ridge between two rivers, where the Irish dhá is represented by a in the present names. In Cloonederown, Galway - the meadow between two rivers - there is no representative of the dha, though it exists in the Irish name; and a like remark applies to Ballyederown (the townland between two rivers), an old castle situate in the angle where the rivers Funshion and Araglin in Cork mingle their waters. Coracow in the parish of Killaha, Kerry, is a name much shortened from its original Comhrac-dhá-inbhir, the large rushy place between two river mouths, otherwise called Ailbhe or Cluain-Ailbhe (Ailbhe's meadow), now Clonalvy in the county Meath. With glaise (a stream) instead of abhainn, we have Ederdaglass, the name of two townlands in Fermanagh, meaning (a place) between two streams; and Drumederglass in Cavan, the ridge between two streams. Though all trace of da is lost in this name, it is preserved in the Down Survey, where the place is called Drumaderdaglass. Ederdacurragh in Fermanagh, means