Whoa! At the ends of both acts of The Butcher Boy, the Irish Rep’s new musical adapted from Patrick McCabe’s 1992 novel, such unsettling things happen that you’re forced to revisit everything that preceded them, assessing how much was fact, how much was fantasy, and whether or not we should trust our narrating protagonist, Francie Brady (Nicholas Barasch, and we shouldn’t). The Butcher Boy isn’t comforting or reassuring or lovable, and it won’t send you out whistling a happy tune. But, and this puts it ahead of much of the current pack, it isn’t stupid.