Royal pain in the ass

Does anyone know the origin of the phrase "royal pain (in the ass)"?

You've actually split it incorrectly: that should be "(royal) pain in the ass/arse". Anything annoying or irksome can be decribed as being, or giving one, "a pain", often elaborated to "a pain in the neck/back/arm/wherever." "Pain in the ass/arse" became common in the 1960s - we may speculate that it was in used before but was simply considered too rude to set down in print. "Royal" is simply an intensifier, as in similar phrases such as "right royally screwed".