Birnam (Brannan in 1345) most likely reflects braonan, ‘little wet place’. The same element also appears in Loch Bhraoin in Wester Ross – Loch Broom in English.
When Shakespeare wrote his play Macbeth, he was not familiar with the name; his source for the play, Holinshed, spelt the name as Birnane. Shakespeare’s first folio edition of Macbeth spells the name Byrnan and Byrnam, whilst later editions chose the less correct Byrnam or Birnam throughout. The form ending in -m became so famous it seems to have largely displaced the authentic name for the place. The form Braonan however was reinstated in the late twentieth century on the bilingual railway station sign.
The original form may have been remembered locally. One source from around 1900 mentions Birnam Hill as Branac in the saying: Tha ceò air Branac ‘there is mist on Birnam Hill’.
For further research see our database: Birnam