willsuds and myself went out for a spin tonight to search for suitable places and we've some good news and some bad news...
GOOD NEWS: There's a place not too far from Enniskerry that has a nice mix of fire roads and down- and up- hill challenges.
BAD NEWS: The combination of road tires and my (lack of) ability meant that I came home with a few more scratches on my bike than I went out with.
We're not 100% sure of the legality of riding in the place we found but there was no-one around to ask

and the barrier was clearly not intended to stop motorcycles