ONlwSG

foraileach adj. [ˈfɔɾaləx], -[ax], -[ɔx], in the sense ‘imperious’ is derived by Mackay (1897, 92) from Ice. for ‘forward, haughty’, adding ‘[p]erhaps this is the prefix in [SG] forneart [sic]’.

SG fòirneart m. ‘violence etc.’ goes back to EG fornert ‘superior strength or power etc.’, a closed compound consisting of the preposition for- ‘on, over’ + nert ‘strength’, with subsequent lengthening of the stressed vowel in Scottish Gaelic before -rn, except in Strathspey and East Perthshire (Robertson 1906c, 101; SGDS Item 154, Points 180–82, 194–97).

Mackay may intend Ice. fár, cf. ON fár adj. in the sense ‘taciturn, stubborn, cold, unfriendly’ (NO), but there is no phonetic basis for the connection. SG foraileach is from forail m. in the sense ‘command, exhortation’ + the adjectival suffix -ach, and goes back to EG furáil, OG eráil ‘enjoining, urging, instigating etc.’ (eDIL^˄); cf. Ir. furáil, foráil (Ó Dónaill 1977); 

Also foláir, via metathesis.

cf. MacBain (1911; so also McDonald 2009, 354) and MacLennan (1925), s.vv. earail, forail.