Roscoff is the French end of the ferry crossing from Plymouth in England. Brittany Ferries operates the service, with crossings taking around 6 hours overnight or slightly longer during the day. This is the most straightforward way to arrive if you're bringing a car or travelling from southwestern England.
By train, the nearest main-line station is Morlaix, about 27 km to the south, connected to Paris by TGV. Local trains and buses connect Morlaix to Roscoff, though the service is infrequent — check timetables in advance.
By road, Roscoff sits at the end of the N12/E50 corridor from Rennes and Paris. The drive from Paris takes around 5 hours. From Brest it's about an hour.