MTB Trails in Galway

Have a look at:
http://www.coillteoutdoors.ie/ (When you select Galway you'll get a list, including the one just past Oughterard). You could also stop by MBW (Bike Store) in Moycullen as that's their area and they attend a lot of the weekend events.

The woods on the road to Sean-a-Feistin can also be accessed from a mountain trail that starts about half way between Moycullen and Oughterard at Doon East.
http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=moycullen+motor+store,+Moycullen,+Ireland&sll=37.0625,-95.677068&sspn=33.160552,78.662109&ie=UTF8&hq=moycullen+motor+store,&hnear=Moycullen,+County+Galway,+Ireland&ll=53.375495,-9.248085&spn=0.023093,0.076818&z=14
It's a continuation of a left-turn into a group of houses just after a disused lay-by and about 2 km before Kinneavy's pub. It's wll worth finding this road because it brings you onto a rough trail over the moutnain with fantastic views back over Lough Corrib and on the other side you sweep down into isolated woods. After about 5-6kms the entrance to the coillte woods will be on the right.
It's very isolated so would advise not to go alone as you'll not meet another soul.