Good question. "Ir" has the specific meaning of going somewhere, whereas "irse" generally denotes leaving a place. "Él se fue" has the meaning of "he left" while "Él fue" has the meaning of "he went." I hope this helps. A subtle difference, albeit an important one!
"me voy" normally is thought of as "I'm leaving/I'm going" as opposed to "I'm going AWAY" so I think any variation on that means "leaving" instead of going "away" ie me fui = I left, se fue = he/she left"
Also, "espalda" is "back" though, interestingly, I inadvertantly found that (and I am not sure how correct Google Translate is) "shoulderS" plural is translated as "espalda". Maybe because if you have something on both shoulders, it is really on your back?