Since Paris-Montparnasse, the line Paris - Brest serves St-Brieuc, Guingamp, then you can join in most near Paimpol (13 km). Finally the bus line n°7 will drive you to Pleubian-Bourg. More information on Tibus.fr
From Rennes, follow the RN 12 (in direction of Brest), then after St Brieuc, take out in direction of Paimpol/Lanvollon. Then, in direction of Tréguier, and finally in the traffic circle of Kérantour, follow Pleumeur-Gautier/Pleubian.