Translation according to P. W. Joyce:
Kilnalag in Galway, and Kilnalug in Westmeath; Coill-na-lag, wood of the hollows. See Lag, vol. i.p. 431 [reproduced below].
Lug or lag signifies a hollow; when used topographically, it is almost always applied to a hollow in a hill; and lag, lig, leg, and lug, are its most common forms, the first three being more usual in Ulster, and the last in Leinster and Connaught. The word is not so much used in Munster as in the other provinces. There is a place near Balla in Mayo called Lagnamuck, the hollow of the pigs; Lagnaviddoge in the same county signifies the hollow of the plovers. Leg begins the names of about 100 townlands, almost all of them in the northern half of Ireland. The places called Legacurry, Legachory, and Lagacurry, of which there are about a dozen, are all so called from a caldron like pit or hollow, the name being in Irish Lag-a'-choire, the hollow of the core or caldron. When the word terminates names it takes several forms, none differing much from lug; such as Ballinlig, Ballinlug, Ballinluig, Ballylig, and Balylug, all common townland names, signifying the town of the lug or hollow. As this word was applied to a hollow in a mountain, it occasionally happened that the name of the hollow was extended to the mountain itself, as in case of Lugduff over Glendalough in Wicklow, black hollow; and Lugnaquillia, the highest of the Wicklow mountains, which the few old people who still retain the Irish pronunciation in that district, call Lug-na-gcoilleach, the hollow of the cocks, i.e. grouse. The diminutives Lagan and Legan occur very often as townland names, but it is sometimes difficult to separate the latter from liagan, a pillar stone. The river Lagan or Logan, as it is called in the map of escheated estates, 1609, may have taken its name from a "little hollow" on some part of its course; there is a lake in Roscommon called Lough Lagan, the lake of the little hollow; and the townland of Leggandorragh near Raphoe in Donegal, is called in Irish Lagan-dorcha, dark hollow.