The entry in G. Malcolm Laws' British Broadside Ballads Traditional in America says "Pat O'Brien" has been found in Maine and was printed in the Bulletin of the Folk-Song Society of the Northeast as well as in The New Green Mountain Songster. There is a further mention of additional stanzas from Vermont printed in the Union newspaper of Springfield, Massachusetts (Dec 4, 1932) by Helen Hartness Flanders. There are no Irish or British references.

I would think almost certainly that this ballad was written in Ireland and is about a real murder.

All the best
Dan