The closest trail in north Irvine is out to Peter's Canyon by Jamboree, then up toward Irvine Regional Park and the whole series of trails by Santiago Oaks, out to Anaheim Hills and Weir Canyon. Get the "Mountain Biking Orange County" book for a good guide to surrounding trails, and Franko's Map of Orange County Trails, and the list of trails on Geoladders. Good luck, and see you out there.