Park at the marina in Bangor, and you have options either east or west along the coast.
Great information about North Down Coastal Path, including detailed maps, on WalkNI.
Trail Head
NOTE: The trails on this site have been ridden before, but you should not interpret their inclusion as an indication that mountain-biking is officially sanctioned on them. Seeking permission from landowners is your own responsibility, as is conducting yourself in a manner that is courteous, respectful to the environment, and conscious of the safety of others.