On balance, I would plump for "Bawnogue" in the song being used in the general sense, rather than the specific.

The Ó Dónaill dictionary gives 13 meanings for "bán"! The 13th is "talamh bán = fallow land, lea, grass-land", so, that's in agreement with Martin's dictionary! The more common word for "field" is though, "gort".

I have never heard the word "bawnogue" used in Donegal. Is it still in use elsewhere in Ireland?