The local bus meets ferry arrivals and runs between Alopronia and Horio/Kastro (€1.80) every half-hour in August, 10 times daily the rest of summer from 7.45am to 10.45pm, and less frequently at other times. A timetable is posted at the terminus, just inland from the port. One bus daily continues from Kastro to Agios Georgios, and a 7.15pm bus runs to the Manalis Winery, making the return journey at 8.45pm.
If you're out of season and nothing is going on, talk to English-speaking Flora in the minimarket at Alopronia, and she'll help you figure out your options. Her son also runs the petrol station, so she's the person to call if it's closed.