'Bawn' is capitalised because everybody just assumes it's a placename. The name appears in many parts of Ireland and is even more common as part of placenames.I think a lot of the speculation here is off the mark unless someone can come up with evidence that 'bawn' has been used in Hiberno-English as an abstract term for rocky land.

The common meaning of 'Bán' is a grass field, not rocky terrain. As an adjective it can mean 'untilled' or 'wild', not 'rocky'.

'Bawn' also means the enclosed land surrounding a castle. I think it is also means a 'yard' in the north of Ireland.

Also the syntax doesn't look right, if 'bawn' is a specific noun, the definite article 'the' would need to be included. Even poetic license would not allow that omission.

I have a reasonably good command of Irish but I'm not a scholar so I am open to correction. However I am not convinced by the arguments put forward above.

There also seems to be a strong wish to find political meanings in a song that is quite straightforward and apolitical.

This is a song about a hired labourer being exploited by a lazy farmer, there is no reference to a landlord.

Large numbers of Irishmen, both Protestant and Catholic enlisted in the British army in the nineteenth century. It would just have been seen a career opportunity by many and there are numerous references to enlistment in this context in Irish folk songs.

I am referring here to the most commonly sung lyrics. There is also a much different version to be found on the net.