Park in the car park at Marley Park
You'll be on and off the bike a time or two, as not all of the trail is rideable, but it's NEARLY all rideable, the views are superb, and the terrain varied. You can drop off Wicklow Way into Djouce Wood and/or Ballinastoe to make a real go of it if you fancy.
OSI Maps 56 and 62 should keep you right.
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.