According to the Oxford English Dictionary
Bourtree: the Elder-tree ( Sambucus nigra). attributive, as in bourtree-berry, bourtree-bush; bourtree-gun a popgun made of the wood of the Elder, after the pith has been removed.
Now only Scottish and northern dialect.
Etymology: Of uncertain phonetic form, and unknown origin. The plausible derivation <
bore is inconsistent with the earliest and with the dialect forms; derivation <
bower, Scots
bour,
boor (in sense of ‘
arbour’) answers phonetically, but is unlikely with regard to meaning. Compare bountree n.
Bountree: Common name in Scotland of the Elder-tree. Also attributive, as in bountree gun.
Etymology: ? A variant of
bourtree; or a distinct word =
bound-tree , < bound from being planted to mark boundaries.
So it does not appear to be of Gaelic origin.
There are several places called Bourtree or Bourtreebush or similar, all in the Lowlands, and Bourtree occurs in 17 of the volumes of OS Name Books. Other spellings are available.
These are some of them.
https://www.geograph.org.uk/gridref/NO9096http://www.geograph.org.uk/gridref/NO7665http://www.geograph.org.uk/gridref/NO4756http://www.geograph.org.uk/gridref/NS4128http://www.geograph.org.uk/gridref/NS3439http://www.geograph.org.uk/gridref/NS2155but I have not come across Bountree or Bowntree before.