An improved rotary drive sprinkler is provided for rotatably driving a pop-up spray head in a stepwise manner through a full-circle or reversibly within a selected part-circle path. The sprinkler includes a reversing trip mechanism having a pair of spaced-apart stops rotatable with the spray head and defining preset end limits of a part-circle path, wherein a stationary trip wire is engageable with the stops to switch an internal rotary drive assembly, such as a ball drive assembly, between forward- and reverse-drive operation. The trip wire flexes to accommodate forced spray head rotation to a maladjusted position beyond the end limits and, upon subsequent sprinkler operation, rides over ramped surfaces on the stops for automatic return to reversible operation between the reset end limits. Alternatively, the stops can be set in side-by-side relation with their ramped surfaces oriented for the trip wire to ride resiliently thereover without switching of the rotary drive assembly. Positional adjustment of the stops is performed quickly and easily from the exterior of the sprinkler after which a vandal-resistant lock device can be installed to prevent inadvertent or unauthorized stop adjustment. In addition, flow of irrigation water through the sprinkler is regulated to obtain improved control over spray head rotational speed irrespective of water pressure and spray head nozzle size.