The old man loaded a bag full of sand onto the mule.

Literal

Old-man [topic-は] mule [on-に] sand [possessive-の] fully entered bag [object-を] placed.

砂のいっぱい入った袋 is a relative clause: 'a bag that was full of sand.' のせる/載せる ('to load onto') takes the destination (ラバに) and the object (袋を). ラバ ('mule') is relatively uncommon vocabulary.