Depends which side of the city you are on.
North side (best side…ok that is a lie I just came from there) you’ve got Howth…it’s ok, but proper hidden local trails. Most of which I dug. not much altitude.
South side (posh bit)… loads around Three Rock, and I mean LOADS, just go up and explore, two man made trails – Metro 1 and 2.
South West side (knackery bit) .. loads above Tallagh into Masseys and around Hellfire Club, again, natural locals trails.
Clubs to look at:
MAD – Mountain Bike Association Dublin – ace group, all ages, all categories.
EPIC – Used to be a bit DH focused, now a bit of everything.
WORC – Wicklow Off Road Club – my old team…it’s a race team.
Giant – Richie Byrne who left WORC to setup EPIC then left – new club… can’t comment as I’d left.
Expert Cycles – think they do group rides?