Cavehill Trails near Belfast, consist of lots of twisting single track descents, bumps and jumps through the trees. Great descents off the summit to both north and south, including the fantastic "quarry track".
Getting onto the quarry track - summit Cave Hill, and look out over Belfast. To your right, there is a single track path that follows the ridge - follow it. You can see it in the foreground of the pic above, just heading to the left and across the hill - it is a single trail to the left of the walking path in the distance. Top-class, really brilliant.
Another great option (but pretty technical at the top): just behind the Napoleon's Nose (view over Belfast) at the bottom of the steps, there is a black iron marker post. Head over the edge here, but be careful! You can pick your way down if you are confident and have good balance, and once things flatten out, stay to the right, and you are onto a fantastic twisting single track through the trees, pretty much the whole way down to the castle. Very rooty and a bit mucky, so best in the dry.
Be careful on the lower slopes of Cave Hill on the obvious walking paths, as the area, particularly the low section just above Belfast Castle, is a popular place for families / walkers, so safety and courtesy are of the utmost importance.