Hemon (1995:§49)

De Arbres
 49. Nouns referring to the double organs of the body generally
 have a dual, or special plural formed with the aid of daou- for
 masculines, and of di- or div- for feminines: 
 
 lagad 'eye', an daoulagad 'the (two) eyes; 
 skouarn 'ear', an diskouarn, an divskouarn 'the (two) ears'.
 
 Note I: These words in turn may be pluralized: 
 daoulagadoù 'pairs of eyes'.
 
 Note II: Words such as lagad, skouarn, etc., when they do not designate
 the organs of the body, have all ordinary plural: 
 lagadoù 'eyes', (a, spot, of grease on the surface of a liquid)'
 skouarnoù 'handles (of vases)'.
 
 Note III: Note the irregular forms: 
 glin 'knee', an daoulin 'the (two)knees'
 dorn 'hand', an daouarn 'the (two) hands'
 gar 'leg', an divhar 'the (two)legs'.
 

reference

  • Hemon, R. 1975a 1995. Breton Grammar. Translated, adapted, and revised by Michael Everson. Dublin: Everson Gunn Teoranta.