Bit of a drag to go all the way to dublin for some trails, MTBireland is great resource but is mainly dubs based too.
Try Cork MTB – good buch of lads, will help you out no end.
http://mbcc.ie/
Or there is another club in Limerick just up the road (mainly students but they do great parties).
http://www.skynet.ie/~mtbclub/
Pretty sure Rob on there has tried to do all the Reeks and stuff (mainly with his gps) but I'm sure I remember him saying that 80% is bog riddled and lots of hike a bike on marginal trails.
If you like things that are a bit less pedal and a bit more body armour get in touch with 021racing.
http://www.021racing.com/home/
Also cork based.
Oh and I think the newset club based in wexford (a bit up the road tbs), Racing 795. Good laughs and all skill levels.
http://racing795.com/