Well, I don't know that I'd want to pick a fight with MacEdward Leach, even posthumously. Perhaps he knew something I don't. Mind you, Hughes had "the dear knows", and glossed it as "the Ulster equivalent of 'goodness knows'". If he had thought for a moment that fairies were involved, I'm sure he'd have said so.

It does seem to be one of those euphemisms, though. Maybe not "devil" after all. Do we have anyone who is familiar with late 19th / early 20th century Antrim usage who might be able to help?