v1.2
Publishing history:
v1.0: 01/10/24
v1.1: 30/04/25
v1.2: 15/08/25
dreòlan or dreòllan m. [ˈd̪̥ɾ(ɛ)ɔːɫ̪an] and dreolan or dreollan [ˈd̪̥ɾ(ɛ)ɔɫ̪an], gen. -ain -[æɲ], -[ɛɲ]. Henderson (1910, 143, 214) derives SG dreollan, dreòllan 
‘[W]ith close o in the Aird [?referring to the area south of the River Beauly and the northern end of Loch Ness], more often with long o elsewhere’ (ibid., 143).
GPC˄ tentatively derives W drel from OEng. þræl.
A. SG dreòll, dreòl(l)an, dreol(l)an
A1.
dreòll 
Written dreóll in Ó Cuív 1980a, 56.
A2.
(i) dreòlan [d̪̥ɾ(ɛ)ɔːɫ̪an] (HSS 1828: ‘wren; silly person’; McAlpine 1832, s.v. dreolan: [dryôl´-an] ‘wren’; Kennedy 1897, 138: ‘wren’: dreòlain (gen.), (but p. 128) dreolan, Arran; Dwelly 1911: ‘silly, inactive person; dwarf; wren); Mac Farlane 1912: ‘wren’; Clachan 1917, 13: ‘silly trifling person, ninny’; MacLennan 1925, with separate entries for ‘wren’ and ‘silly person; dwarf’; Dieckhoff 1932, s.v. dreolan: [drèòːLan] ‘wren’; Garvie 1999, 64: ‘wren’); also dreôlan (Lhuyd 1700, 133 Item 90: ‘wren’, Argyll), with the circumflex indicating a long vowel;
(ii) dròlan [idem] (Faclan bhon t-Sluagh˄: ‘wren’, Islay), with an unhistorical pronunciation spelling; 
The initial cluster /dr´/ has to all intents and purposes fallen together with /dr/ in Scottish Gaelic: by the second half of the 20th century, /dr´/ is restricted to St Kilda (SGDS Points 14–15), part of North Uist (17–19), part of Barra (28–30) and part of Islay (56), while extremely weak palatalisation is found within the Skye and Lochalsh area (100–118); see SGDS Items 345–347, similarly for tr-, see Items 863, 865, 867–869.
(iii) dreòllan [idem] (MacLeod and Dewar 1839: ‘wren; silly inactive person’; MacEachen 1842: ‘wren; dwarf’; Clachan 1917, 13: ‘silly trifling person, ninny’), with -ll- perhaps based on droll etc. (B and C), but at any rate not phonetically distinct from -l-, above.
A3.
(i) dreolan [d̪̥ɾ(ɛ)ɔɫ̪an] 
But note Henderson’s (1910, 143) description with ‘close o’ (fn 1, above).
(ii) dreollan [idem] (Mac Farlan 1795: ‘silly person’; MacFarlane 1815: ‘idem’; Armstrong 1825: ‘wren; silly person, ninny’; Forbes 1905, 347: ‘wren’; AFB˄: /drɔLan/, with separate entries for ‘wren’ (Ardnamurchan, Mull, Colonsay, Islay, Jura, Gigha, Kintyre, Arran) and ‘lazy/idle person, sluggard, slouch(er), layabout, idler, lazybones; daydream; silly person’); also Shaw (1780), but who does not use lengthmarks on headwords;
(iii) dreollin (Forbes 1905, 347: ‘wren’): Forbes’s source is unknown; if dreollin really indicates a pronunciation *[d̪̥ɾɔɫ̪in] or *[d̪̥ɾɔlin], cf. SG cailin ‘maiden, young woman’ (EG cailín, with the suffix -ín).
B. SG droll, dreoll
(i) droll [d̪̥ɾɔuɫ̪], (Argyll) [d̪̥ɾɔɫ̪ː] (MacFarlane 1815: ‘sumph’; Armstrong 1825: ‘idle inactive person’; HSS 1828: ‘lazy idiot, sluggard’; MacLeod and Dewar 1839: ‘awkward sluggard’; MacEachen 1842: ‘sluggard’; Dwelly 1911: ‘awkward sluggard, idle inactive person’; Clachan 1917, 13: ‘awkward sluggard’; MacLennan 1925: ‘sluggard’);
(ii) dreoll [idem] (HSS 1828: ‘slubberdegullion, homo torpidus’; MacLeod and Dewar 1839: ‘paltry sorry fellow’; Dwelly 1911: ‘paltry worthless fellow’; Clachan 1917, 13: ‘idem’); cf. similar confusion between dreòlan and dròlan (A2 (ii)).
C. SG drollaire
(HSS 1828: ‘sluggard, saunterer’; MacLeod and Dewar 1839: ‘sluggard, saunterer; awkward clown’; MacEachen 1842: ‘sluggard’; Clachan 1917, 13: ‘awkward sluggard’; MacLennan 1925: ‘lazy fellow, sluggard’).
D. SG dreallaire, dreallan
(i) dreallaire (HSS 1828: ‘idler, saunterer’; McAlpine 1832: ‘loiterer’; MacLeod and Dewar 1839: ‘idler, saunterer, loiterer’; Clachan 1917, 13: ‘idler, lounger’; MacLennan 1925: ‘loiterer’);
(ii) dreallan (MacEachen 1842: ‘dreòllan “a wren, a dwarf”, rather dreallan’; Faclan bhon t-Sluagh˄: ‘loiterer’, Skye).
Ir. dreolán, dreoilín (Ó Dónaill 1977), dreóilín (de Bhaldraithe 1977, 134), dreóillín (Mhac an Fhailigh 1980, 72–73, line 121) etc.
For further Irish references, see Ó Baoill 1978, 39–40, and Ó Cuív 1980a, 56.
SG dreathan (occasionally dreadhan) is from disyllabic EG dreän, dréën, with -th- (-dh-) marking hiatus in the Scottish Gaelic form.
SND˄ derives Scots drollan ‘half-witted person’ from SG dreòlan.
SG droll (B (i)) ‘sluggard, loiterer’ may derive from Scots drail, dral(l) [dra(:)l] ‘a dawdler over work, a poor, incompetent sort of worker’ (SND˄), which, with a ~ o alternation in Gaelic, would give SG [d̪̥ɾɔuɫ̪], (Argyll) [d̪̥ɾɔɫ̪ː] regularly; so SG drollaire (C), with the agentive suffix -aire. The variant spelling dreoll (B (ii)) may have arisen through confusion similar to dreòlan ~ dròlan (A2 (ii)), as a result of the loss of a distinction between /dr´/ and /dr/ in Scottish Gaelic (fn 4, above).
The stressed vowel of SG dreallaire and dreallan may have arisen through confusion with dreathan, but compare also the alternation in dreòlan-teasbhuidh ~ dreallan-teasbhuidh ‘grasshopper’ and in dreoll ~ dreall ~ droll in the sense ‘door-bar’ (Dwelly 1911).