I believe you would have to go from Jarabacoa to La Vega by publico, then by publico to San Franciso de Marcoris then catch the Caribe Tours bus to Samana. I could be wrong, so here is the Caribe tours site. Phone 809-221-4422
Contáctenos
Don't know if this is accurate. : From Santiago:
"Transporte PEPE" Tel:582-2134 or 582-5709 (Spanish only) has buses daily to Samaná, departing Santiago at 8:00a.m., 9:30a.m., 1:15 p.m. and 2:30p.m. from the bus stop on Pedro Francisco Bonó street, one block from San Luis.
One direct bus to Las Terrenas at 3:00p.m.
Bus driver will stop on request, but will not stop to pick up more passangers on the route to Samana.