The best thing to do is get in touch with people and keep in touch.  There are a number of gaming clubs about the city, most of which are more than happy to accept new members.  Some of these clubs will have people who will be starting games soon, while others will have existing games that may have spots available. 

You should definately go to the merc some nights and get in contact with people directly, join in a few board games or similar and get to know people: people generally are more likely to play an RPG with someone they know.

In addition to the Merc gaming nights, there is a similar games night run in Cassidey's of Westmoreland Street on Thursdays by the IGA.  As this is currently getting started, you'll stand a better chance of finding a new-starting game in there than most other places.

You could also contact the igaming@yahoogroups.com mailing list, as there should be someone there who might be able to help you further.

The various college societies are largely shutting up shop for the summer, but some of them may be able to help you a bit over the summer.  Trinity Gamers maintains a forum over the summer and has limited opening hours.  The members (myself included) still game during the summer and are generally happy to see new people joining up.  If you go on the forum http://societies.csc.tcd.ie/~gamers/forum and ask about new games you should be able to get some help.

Hope that's of help,
Oisín