This is just my opinion, but I think you should include Irish Language songs if the story is close enough to the Child Ballad.

I also think that you might as well include written down texts, especially if there's no sound recording available of that particular ballad.
That way it'll be a complete overview of all the Child Ballads known to have been sung in Ireland.(And hopefully get people to sing them again)

I have a question, are you going to include a single "definite" version of each Child Ballad or will you include multiple versions where available?
Anyways, I wish you best of luck with your project, I'm looking forward to seeing/hearing it when it's completed.