Adaptations are tricky. The best ones manage to cover the key points and honor the tone of the source material without getting bogged down by too many narrative details. These elements must also work well with the tools of the chosen medium. The Butcher Boy, a new musical now playing at Irish Repertory Theatre, achieves the benchmarks of a solid, imaginative adaptation, despite the oddball choice of transforming Patrick McCabe’s acclaimed 1992 novel about a schoolboy’s descent into mischief, madness and murder as the world around him collapses in a small Irish town in the 1960s.  The Butcher Boy features a