ONlwSG

dail , gender unknown, [ˈd̪̥al] ‘valley’.

SG dail in the sense ‘valley’ is sometimes confused with SG dail in the sense ‘(water-)meadow, field’ (e.g. MacBain 1911; MacLennan 1925; Stewart 2004, 409; McDonald 2009, 348; 2015a, 156). The latter is a loan from Pict. *dol (Watson 1926, 414–19; Jackson 1969, 149; 1983, 152: W or Pict. dol).

ON dali, dat. of dalr m. ‘valley’, yields SG [ˈd̪̥al] regularly, with apocope in Gaelic. The loan-word is attested only in Lewis: 

As an element in Old Norse loan-names in Scottish Gaelic, e.g. SG Dìobadal < CSc. *Diupadal acc. ‘(the) deep valley’ (Cox 2022, 677–80), ON dalr is common (e.g. MacBain 1895, 224; Watson 1904, 265–66; 1906, 362; Henderson 1910, 214; Fraser 1979, 20– 21) but in such a context does not constitute a loan-word.

Borgstrøm notes its occasional use in Bernera, 

In the sense ‘long, narrow valley’ (in Oftedal 1954, 374).

while on the West Side Cox records the word as an element in Gaelic originated place-names: (1) Dail Geannain NB2036 ‘the valley of *Geannan’ 

?< SG geannan ‘wedge-shaped place’.

(Cox 2002a, 263), and (2) the settlement names Daile Beag [ˌd̪̥alə ˈb̥eɡ̊] NB231454, gen. Dhaile Beaga [ˌɣalə ˈb̥eɡ̊ə], and Daile Mòr [ˌd̪̥alə ˈmoːɾ] NB219445, gen. Dhaile Mòire [ˌɣalə ˈmoːɾʲ(ə)] (Cox ibid.; 1991, 492; 1992, 139; 2022, 675–78). The form daile is analysed as consisting of an older, vocalic Gaelic plural form that would have originally denoted the valleys collectively, and to which originally plural forms of the adjectives beag ‘small’ and mòr ‘large’ were later added, although singular forms of the adjectives are now used in the radical case.

SG Na Dailean, with the more recent Gaelic plural ending -an, is now the collective name for the two settlements. (Note that the plural ending shows no trace of a consonantal stem; contrast dailthean (e.g. Dieckhoff 1932), pl. of dail ‘(water-)meadow etc.’ (see fn 1, above), gen. dalach.)

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The settlement name SG Dail (Eng. Dell) in the north of Lewis is a loan-name from ON *Dali dat. ‘(the) valley’ (Cox 2022, 673–76).