|Sources||The Gaelic is Am Preas (pronounced press), “the thicket”, or rather “clump of trees”. F. C. Diack, Some Local Place-names, Aberdeen Free Press 31.8.1910
parish of B. sgir bhreis or bhraois i.e. vresh or vraosh ‘Sgir bhrish; tha mi dol do Bhrish.’ Diack MS2276
Preas: Watson in Dwelly
Sgire Bhréis: Watson CW9 [Watson quoting Diack]
The name of this parish in the old documents is written as Brass. That form in Sc. speech would be pronounced Bress (as Clatt is pronounced Clett), whence the present Birse by normal transposition. The first Gaelic sound- noted above, pres, looks like preas, a wood, which with the def. article is am preas, giving the sound Bress.: Inscriptions of Pictland [MacDonald’s comments to Diack’s evidence – almost identical comment in 1952, 176]
There seem to be two competing forms at work (Am) Preas and Br(ao)is. The proposed form was chosen as it derives from Diack’s MS which relates to actual usage rather than discussion of the etymology. Watson’s form with the open [e] suggests he heard rather than read Diack’s pronunciation.