I’m not a Dublin local, I’m sure someone on here is, but I’ve ridden Two Rock/Three Rock on the South side of Dublin a few times. Between there and Djouce. There are some great trails to be had. They hold a round of the Irish Gravity Enduro league there every year, so there are plenty of trails to be found. They might have a detailed map at the Biking.ie rental store. Maybe ask if you can get a copy of the race map from last year or something. (I’ll have a look see if I can’t track down mine to send you a scan) Otherwise, this is all I’ve found;
Full map from Coilte Website
[Edit] Or pop over to the MadMTB Forums, they’re a sound bunch of bikers who regularly ride Ticknock and Ballinastoe.